V.I. Revisited

[buster]

Link


Quote:
In summary, the ASTM Viscosity Index is widely used as a measure of "oil quality." However, the rating method suffers a number of inconsistencies that penalize lower viscosities oils compared to their higher viscosity counterparts. In addition, V.I. has no fundamental relation to the true viscosity-temperature behavior of an oil, even though it is widely presumed to have one. At best, V.I. is a very rough guide to viscosity-temperature behavior.

 

[dailydriver]

CATERHAM is going to have a FIT over this post!!!!!!

 

[buster]

I didn't read the entire thing yet. Just skimmed over it. The quote I posted caught my attention.

 

[Roob]

I didn't bookmark the link from CATERHAM's post on another thread regarding VI.
However I did copy the article. Enjoy.

Viscosity importance.

Too often the viscosity index (VI) is disregarded as a lubricant selection parameter. One reason is simply because it is poorly understood. Some people think the viscosity index is incorporated in the ISO Viscosity Grade, but it is not. It stands alone as an independent lubricant performance differentiator.

We all know that viscosity is the single most important physical property of a lubricant. It is a crude measure of a lubricant’s molecular constitution from the standpoint of hydrocarbon chain size. Viscosity is determined by creating friction between the molecules from fluid movement. The higher the intermolecular friction (longer molecular chains), the higher the viscosity.

Viscosity determines film thickness and film strength in machines. It also influences other important factors such as those in the table below.


The Effects of Viscosity

However, a lubricant’s viscosity is meaningless unless the temperature is noted, i.e., the temperature at which viscosity is measured. Conversely, machines don’t care about temperature as it relates to viscosity (loosely speaking). Minimum, maximum and optimum viscosity requirements demanded by machines don’t take temperature into account. Instead, a machine’s viscosity requirement is based on such things as component design (e.g., bearing), loads and speeds. You may recall that the well-known Stribeck curve doesn’t have a temperature variable.

That said, because machines require a certain viscosity, and temperature is known to have a dramatic influence on viscosity, it is imperative that you take the average operating temperature and temperature range into account when selecting viscosity. Remember, the ISO Viscosity Grade system only reports viscosity at a single temperature: 40 degrees C. Most importantly, for any candidate oil, you need to know how viscosity changes relative to a change in temperature.

The viscosity index was developed for this purpose (ASTM D2270) by E. Dean and G. Davis in 1929. It is an empirically derived, unitless number. Based on the methodology, Pennsylvania crude (paraffinic) was set as a benchmark at one extreme, representing low viscosity changeability relative to temperature. At the other extreme was Texas Gulf crudes (naphthenic). If a lubricant was similar to the Pennsylvania crude, it was assigned a VI of 100. If it was similar to Texas Gulf crude, it was assigned a VI of 0. Halfway in between was a VI of 50, and so forth. The higher the VI, the more stable the viscosity across a range of temperatures (more desirable). The temperatures used to determine the VI are 40 degrees C to 100 degrees C.

An Internet search on “viscosity index calculator” will direct you to several Web pages. You can use these calculators in a variety of ways. For instance, if you know the viscosity of a lubricant at two different temperatures, you can use the calculator to give you an estimation of the VI. Better yet, you can enter a single known viscosity (and temperature) and VI (commonly found on a lubricant’s product data sheet) to calculate the viscosity of the same lubricant at any other temperature (say, a machine’s operating temperature).

Lubricants with VIs as low as minus 60 are available today. Other lubricants can have VIs ranging over 400. However, the vast majority of lubricants on the market will have VIs in the range of 90 to 160.

Let’s take a look at two different oils that share one common property - they’re both ISO VG 150. However, one of these oils (Oil A) has a VI of 95 (mineral oil), while the other (Oil B) has a VI of 150 (synthetic). Now let’s examine the viscosity of these oils from minus 20 degrees C (minus 4 degrees F) to 100 degrees C (212 degrees F). This is shown in the table below.

89% of lubrication professionals consider an oil’s viscosity index when selecting a lubricant, according to a recent survey at machinerylubrication.com
Comparing the 95 VI oil to the 150 VI oil, there is a 236-percent difference in viscosity at minus 20 degrees C and a minus 25-percent difference at 100 degrees C. Of course, there is no difference at 40 degrees C. As mentioned previously, the machine dictates the viscosity requirements based on its design and operating conditions. These conditions influence temperature, which in turn influences viscosity, which influences the protection provided.

Many machines use a common lubricant across numerous frictional zones and have varying loads, speeds and temperatures. Outdoor mobile equipment typically works under these challenging conditions. For such machines, there is no easy way to identify optimum viscosity by a theoretical calculation. Instead, the ideal viscosity is estimated using actual field measurements by applying a trial-and-error approach (simply testing with different viscosity oils and measuring temperature and wear protection).




Comparison of Two Different Oils with the Same Viscosity Grade



For machines of constant load, constant speed and constant ambient temperatures, the ideal viscosity very often results in the lowest stabilized oil temperature. Oils of lower or higher viscosities (than the optimum viscosity) will typically increase the oil’s stabilized temperature due to either churning losses (too much viscosity) or mechanical friction (too little viscosity).

If conditions are not constant (variable loads, variable speeds, variable ambient temperatures, etc.), then there is a need for not only the optimum viscosity but also a high viscosity index to stabilize the optimum viscosity. The more variable the conditions, the greater the need for high VI oils.

Also, keep in mind that for a great many machines there has been no past experimental or theoretical effort to identify the optimum viscosity. Viscosity selection is more of a wild guess. This too calls for a high VI lubricant.







You must also consider that the ISO Viscosity Grade system (ISO 3448) is based on 50-percent increments between grades. As such, if you go from ISO VG 100 to VG 150, it is a 50-percent jump. When the viscosity options are all 50 percent apart, it is difficult to achieve precision lubricant selection. These large viscosity steps are further magnified at lower temperatures. This is yet another reason to select high VI lubricants.



High or Low Viscosity Index?

A lubricant may merit having a high VI for one or more of the following reasons:

The optimum viscosity is not known
Varying loads and speeds exist
Varying ambient temperatures exist
To boost energy efficiency
To boost oil service life (lower average temperature)
To boost machine service life (fewer repairs and downtime)
Cheaper, lower VI lubricants may make sense if:

Speeds and loads are constant
Temperature is constant (constant ambient temperature or a heat exchanger is in use)
The optimum viscosity at the operating temperature is known and is consistently achieved


An oil’s VI can also tell you useful information about a lubricant’s formulation, including the type and quality of base oils. For instance, highly refined and pure mineral oils will have correspondingly higher VIs. Certain additives, such as viscosity-index improvers and pour-point depressants, influence VI as well. Remember, oils loaded with VI improvers, especially of certain types, are prone to permanent loss of VI and viscosity over time. There are ASTM tests for measuring the VI stability of lubricants exposed to high shear.

Viscosity index values are on almost all product data sheets for commercially available lubricants. This simple number is there for a reason and should be strongly considered when writing a specification for lubricants in nearly all machine applications.

.

Machinery Lubrication (6/2012)

About the Author

Jim Fitch
Jim Fitch, a founder and president of Noria Corporation, has a wealth of experience in lubrication, oil analysis, and machinery failure investigations. He has advised hundreds of companies on .

 

[Jeffs2006EvoIX]

This is going to be a GOOD one.

So it begins

 

[Gabe]

I am with Buster on this one.

To often, members on here will become fixated on a single number to represent how good an oil is. Currently it is VI. Before that, it was TBN.

 

[wally6934]

Originally Posted By: Gabe
I am with Buster on this one.

To often, members on here will become fixated on a single number to represent how good an oil is. Currently it is VI. Before that, it was TBN.


I have been copying many of Caterham’s posts in the last few months for future reference since it became clear to me he is one of the few people on this board that actually knows what he is talking about. Because of this it was fairly easy for me to find his quote and compare them to the article.

From the article.

“We all know that viscosity is the single most important physical property of a lubricant.”

Caterham says.

“If you can buy a lower VI oil for less, you live in a hot climate like Arizona and you do mostly highway driving, you're minimizing the disadvantages of the lower VI lube but don't think for a second that it's a better lube at least as it concerns an oil's viscosity characteristics. Yes VI only pertains to the issue of an oil's viscosity and there are other important aspects of an oil, but viscosity is THE MOST important aspect of an oil and what we always consider first.”

Well it looks to me that Caterham was right, again. Let’s see if any of the wannabes are man enough to apologize.

 

[martinq]

Originally Posted By: wally6934
Well it looks to me that Caterham was right, again.

Wait, I'm confused ... hasn't this always been the case?!?

 

[Shannow]

Viscosity is what keeps your bearing surfaces apart...it is the single most important parameter in "hydrodynamic" lubrication...viscosity index is not.

If the oil is optimised at the operating point, as all designs attempt to do, at best VI will have less drag during warm-up. Not less wear, less drag.

Chasing VI means not choosing any oil that has a 5W or 10W in front of it, which costs more on the purchase price, and in the absence of decreased wear (hydrodynamic is what it is), may or may not be paid for in warm-up fuel savings.

As to starting performance, that's what the "W" ratings are there for, and I haven't seen VI mentioned there either.

 

[rikstaker]

 

[buster]

Originally Posted By: Shannow
Viscosity is what keeps your bearing surfaces apart...it is the single most important parameter in "hydrodynamic" lubrication...viscosity index is not.

If the oil is optimised at the operating point, as all designs attempt to do, at best VI will have less drag during warm-up. Not less wear, less drag.

Chasing VI means not choosing any oil that has a 5W or 10W in front of it, which costs more on the purchase price, and in the absence of decreased wear (hydrodynamic is what it is), may or may not be paid for in warm-up fuel savings.

As to starting performance, that's what the "W" ratings are there for, and I haven't seen VI mentioned there either.


+1

 

[Tom NJ]

Originally Posted By: Shannow
Viscosity is what keeps your bearing surfaces apart...it is the single most important parameter in "hydrodynamic" lubrication...viscosity index is not.

If the oil is optimised at the operating point, as all designs attempt to do, at best VI will have less drag during warm-up. Not less wear, less drag.

Chasing VI means not choosing any oil that has a 5W or 10W in front of it, which costs more on the purchase price, and in the absence of decreased wear (hydrodynamic is what it is), may or may not be paid for in warm-up fuel savings.

As to starting performance, that's what the "W" ratings are there for, and I haven't seen VI mentioned there either.


Well stated!

Tom NJ

 

[buster]

Originally Posted By: Gabe
I am with Buster on this one.

To often, members on here will become fixated on a single number to represent how good an oil is. Currently it is VI. Before that, it was TBN.


True. And we've seen that TBN retention and oxidation control is more important than starting TBN.

This is why products need to go through extensive testing to validate performance claims because otherwise you're just going by word of mouth. Many companies choose to play the numbers game.

 

[Garak]

I think we have to be a little cautious here. CATERHAM does like his high VI stuff, but anyone who's been paying attention know he doesn't just grab the highest VI available and run it without question. And while he does like to point out when a specific example has a high VI, we have to remember that he generally uses the number when he's comparing oils of the same grade. He points out that PP 5w-30 has a higher VI than PU 5w-30. Or, he mentions VI when the implications are pretty darned obvious - a 10w-30 SN/GF-5 will have a lower VI than a 5w-30 or a 0w-30 in SN/GF-5.

I don't chase a high VI, but I know the value in a high VI. Like any other measure of an oil's property, VI has its limitations. For example, the cold cranking specification means a good deal to me. I bet Shannow doesn't care much about it. And we already know the limitations of the SAE grading system. Additionally, as CATERHAM has pointed out, KV has limited utility, KV graphs are inherently out to lunch as temperature approaches freezing (and goes lower still), and VI by extension has limited utility at cold cranking temperatures. I haven't seen him suggest a high VI oil for cold cranking at the expense of MRV when someone actually has a use for an oil with impressive cold cranking properties.

There's another thing that's absolutely crucial here. The measure of VI is not exactly the same as the concept of VI. Of course the measure has its limitations. But, the concept of an oil that retains its viscosity properties in extreme cold and extreme heat should not be lost on anyone here.

 

[Solarent]

The Article Buster used to start this thread is very instructive when it comes to how VI is calculated, and how it relates to base oils.

We have discussed many times, and I have been meaning to start a conversation on this topic for a while, but have only recently put it into writing.

Unfortunately it will make for a big post, but since it relates to the article buster linked to and some of the other comments I think I will put it here....

 

[Solarent]

There are some members who believe that Viscosity Index trumps almost any other characteristic of a finished engine oil, there are others who claim that VI is difficult to rely on due to the nature of it's calculated value, it's arbitrary rating scale and the undue influence of Viscosity Index Improvers (VII's) on the final rating of the oil.

Who is right? Is Viscosity Index the holy grail of oil specifications? or is the Viscosity Index less important than the other specifications and information we can find out about through VOA or other means?

My answer is yes.... and yes. You are both right. And here is why:

To understand how important VI is, one needs to recognize the role it plays in base stock selection and how it figures into oil formulations. There is a BIG difference in the VI of a base stock, and the VI of a finished lubricant - and both are very important answering the question.

BASIC DEFINITION: The VI of an oil is calculated based on the change in flow rate as the temperature changes, so if you use the ISO scale as a reference point:
Oil A ISO 46
@40 C: 46 cSt
@100 C: 5.8 cSt

Oil B ISO 46
@40 C: 46 cSt
@100 C: 8.2 cSt

Oil B will have a higher VI. This is because the oil does not thin out as much as the temperature increases. If these oils were measured at 0 C, Oil B would be thinner than Oil A. When two oils who have the same viscosity at any given temperature(ie room temperature), the one with the higher VI will resist thickening as the temperature drops AND will also resist thinning as the temperature increases. (this means that the higher the VI the less change in flow rate as the temperature changes)

BASESTOCKS: When creating basestocks, the VI is affected by the type of feedstock and through the processing techniques used. It is generally assumed that by using the same kind of feedstock, and going through the same distillation or hydrotreating process will consistently result in a basestock with the same VI (with some exceptions as the viscosity gets below 7.3 cSt @ 100 C). This is very important because the API categorizes basestocks by their feedstock and VI.

In today's engine oils, the defining characteristics between a conventional oil and a synthetic is the API Group of the base stock used. Following the API definitions as well as the acceptance of Group III, VI and V oils as synthetic, then this means that Viscosity index is the ONLY baseoil characteristic separating conventional oil and synthetic. Simply put if the base oils used each has a VI of 120 or higher it is synthetic (with some minor exceptions), if not it is conventional.

As such, the conventional vs synthetic debate - which we have all the time comes down to this very simple question: Do you want an oil which is going to maintain a more consistent viscosity as the oil temperature changes? If the answer is yes, then go with synthetic, if no then conventional is fine.

Also, if you are choosing to use synthetic based oils, then you are in fact saying that VI is the most important oil specification.

Now, there are other benefits to using synthetic bases, such as volatility, the ability to work with certain additive formulations etc, but this is much more dependent on the type of synthetic used (Gr III, Gr III+, PAO-Gr IV, or the catch all other category (Gr V). But you're never going to know exactly what base is used or in what quantity unless you have an in with the company who made the formula. Also, all of these performance characteristics (including VI) can be influenced by the additives used in the finished oil. So hypothetically I could use Gr II stocks with the right additives to create an engine oil that rivals a synthetic with the wrong additives. (In reality, this just doesn't make sense because it would be expensive, and you wouldn't be able to charge the extra premium for "synthetic" oil).


FINISHED FORMULAS: How the Viscosity Index works with a finished oil is a little bit more complicated then dealing just with base stocks. There are 3 primary reasons for this:

1-Finished oils typically use 2-3 different base stocks from multiple base oil categories depending on what SAE grade the oil needs to be.

2-Pour point depressants - this additive is used to alter the oil's thickening characteristics as the temperature drops, they work by altering the structure of the wax crystals which form as the oil thickens.

3-Viscosity Index Improvers - these additives affect how the finished oil flows by expanding and contracting as the temperature changes. As the temperature increases they expand, causing the oil to resist thinning, as the temperature decreases they contract,thereby reducing their influence on the flow rate of the oils.

Creating a 5W30 - the target viscosity for a 5W30 oil requires a KV@100C of 9.3-12.5 cSt with a max Cold Cranking Viscosity of max. 6600 cP @ -30C and Cold Pumping viscosity of max. 60 000 cP @ -35 C
To reach these targets, formulators will mix 2-3 base oils with KV@100C ranging from 4-8.5 cSt (about 50% of the formula will likely be in the 6 cSt range) with the correct blend of additives to reach the final target performance at the required temperatures. Because these targets are a range, there is plenty of room to create multiple oils with many Viscosity Indexes, depending on what ratios are used. Generally, a higher concentration the thinner the base oils (which are used to improve low temperature performance), requires an increase in concentration of VII's to ensure proper viscosity at operating temperature.
**You may notice that in the range of base oils, ALL of them are lower then the target 9.3-12.5 cSt. This is because the operational viscosity of most oils - conventionals and synthetics rely heavily on VII's (between 5-10% of the formula) to reach the necessary targets while maintaining the proper cold temperature performance.

So, if you are comparing two 5W30 oils, and one has a higher VI, then it will maintain it's flow rate better as the temperature changes. This could be due to a higher concentration of VII's, the use of High VI base stocks, or a combination of both. The bottom line is that in cold temperatures the oil with the higher VI will generally be thinner for better flow, and at high temperatures the one with the higher VI will thicker and able to better maintain hydrodynamic lubrication. So if you are concerned with maintaining the right flow rate (viscosity) oil for your engine and your operating conditions, then the VI is the most important oil specification.

A word of caution: be careful trying to predict an oil's viscosity at extremely cold or extremely hot temperatures using the VI as a reference. Because the VI is a calculated number, which uses a somewhat arbitrary formula, you can get widely different results due to the differences in formulas of the finished oils. Most online calculators can give an estimate, but this may vary because the components of the finished oil's formula may be designed to produce a specific result, not just based on the calculated VI.


BUT WAIT, THERE'S MORE!
Unfortunately, an engine isn't always at hydrodynamic lubrication, in fact, a typical engine is operating at various levels of boundary, mixed, and hydrodynamic regimes in multiple points of the engine at the same time. Finished oils are designed not based on viscosity performance alone anymore. (15-20 years ago it was one of the most important considerations) Now engine oils are also about film strength, sacrificial additive layers, detergency, and other important characteristics. In many cases where thinner oils are being used - to improve fuel economy - oils are coming with better concentrations of additives to prevent wear in boundary and mixed lubrication regimes. Bearing Materials are also being improved to prevent wear in boundary and mixed lubrication conditions. Unfortunately there is no standardized data which appears on the oil's technical data sheet which can tell us about the wear protection. It's not like the oil manufacturers publish the results of the engine sequence tests. Without this information it is impossible to quantify the difference in any oil's ability to protect an engine. On top of that, operating conditions have lot to due with how an engine oil/engine combination performs, so without extensive UOA and other testing comparison is really just a guessing game.

So why is VI the holy grail of oil specification: Matching the right viscosity properties of an oil to your engine is one of the important considerations in selecting the best oil for your vehicle. It will continue to be very important when the next round of oil specifications are completed, as new engines will be designed for thinner oils, and the new thinner oils (starting in 2016) may not work for many older engines. As far as comparing the viscosity performance of two oils goes, the VI is king - as long as you remember that the VI comes from a combination of baseoil selection and additive formulation.

Why is the VI not the holy grail? there is much more to an oil's performance in an engine than just the viscosity performance. But most of the information you need to compare additive package performance isn't available on your typical specification sheet, mostly because there aren't bench tests that can be used to accurately quantify that information. So through some experimentation and work on your own vehicle (if you care and have the time to do so), you can find a great oil/vehicle combination that suits your needs, and it may not necessarily need to be the one with the highest VI.

 

[Clevy]

Originally Posted By: Solarent
There are some members who believe that Viscosity Index trumps almost any other characteristic of a finished engine oil, there are others who claim that VI is difficult to rely on due to the nature of it's calculated value, it's arbitrary rating scale and the undue influence of Viscosity Index Improvers (VII's) on the final rating of the oil.

Who is right? Is Viscosity Index the holy grail of oil specifications? or is the Viscosity Index less important than the other specifications and information we can find out about through VOA or other means?

My answer is yes.... and yes. You are both right. And here is why:

To understand how important VI is, one needs to recognize the role it plays in base stock selection and how it figures into oil formulations. There is a BIG difference in the VI of a base stock, and the VI of a finished lubricant - and both are very important answering the question.

BASIC DEFINITION: The VI of an oil is calculated based on the change in flow rate as the temperature changes, so if you use the ISO scale as a reference point:
Oil A ISO 46
@40 C: 46 cSt
@100 C: 5.8 cSt

Oil B ISO 46
@40 C: 46 cSt
@100 C: 8.2 cSt

Oil B will have a higher VI. This is because the oil does not thin out as much as the temperature increases. If these oils were measured at 0 C, Oil B would be thinner than Oil A. When two oils who have the same viscosity at any given temperature(ie room temperature), the one with the higher VI will resist thickening as the temperature drops AND will also resist thinning as the temperature increases. (this means that the higher the VI the less change in flow rate as the temperature changes)

BASESTOCKS: When creating basestocks, the VI is affected by the type of feedstock and through the processing techniques used. It is generally assumed that by using the same kind of feedstock, and going through the same distillation or hydrotreating process will consistently result in a basestock with the same VI (with some exceptions as the viscosity gets below 7.3 cSt @ 100 C). This is very important because the API categorizes basestocks by their feedstock and VI.

In today's engine oils, the defining characteristics between a conventional oil and a synthetic is the API Group of the base stock used. Following the API definitions as well as the acceptance of Group III, VI and V oils as synthetic, then this means that Viscosity index is the ONLY baseoil characteristic separating conventional oil and synthetic. Simply put if the base oils used each has a VI of 120 or higher it is synthetic (with some minor exceptions), if not it is conventional.

As such, the conventional vs synthetic debate - which we have all the time comes down to this very simple question: Do you want an oil which is going to maintain a more consistent viscosity as the oil temperature changes? If the answer is yes, then go with synthetic, if no then conventional is fine.

Also, if you are choosing to use synthetic based oils, then you are in fact saying that VI is the most important oil specification.

Now, there are other benefits to using synthetic bases, such as volatility, the ability to work with certain additive formulations etc, but this is much more dependent on the type of synthetic used (Gr III, Gr III+, PAO-Gr IV, or the catch all other category (Gr V). But you're never going to know exactly what base is used or in what quantity unless you have an in with the company who made the formula. Also, all of these performance characteristics (including VI) can be influenced by the additives used in the finished oil. So hypothetically I could use Gr II stocks with the right additives to create an engine oil that rivals a synthetic with the wrong additives. (In reality, this just doesn't make sense because it would be expensive, and you wouldn't be able to charge the extra premium for "synthetic" oil).


FINISHED FORMULAS: How the Viscosity Index works with a finished oil is a little bit more complicated then dealing just with base stocks. There are 3 primary reasons for this:

1-Finished oils typically use 2-3 different base stocks from multiple base oil categories depending on what SAE grade the oil needs to be.

2-Pour point depressants - this additive is used to alter the oil's thickening characteristics as the temperature drops, they work by altering the structure of the wax crystals which form as the oil thickens.

3-Viscosity Index Improvers - these additives affect how the finished oil flows by expanding and contracting as the temperature changes. As the temperature increases they expand, causing the oil to resist thinning, as the temperature decreases they contract,thereby reducing their influence on the flow rate of the oils.

Creating a 5W30 - the target viscosity for a 5W30 oil requires a KV@100C of 9.3-12.5 cSt with a max Cold Cranking Viscosity of max. 6600 cP @ -30C and Cold Pumping viscosity of max. 60 000 cP @ -35 C
To reach these targets, formulators will mix 2-3 base oils with KV@100C ranging from 4-8.5 cSt (about 50% of the formula will likely be in the 6 cSt range) with the correct blend of additives to reach the final target performance at the required temperatures. Because these targets are a range, there is plenty of room to create multiple oils with many Viscosity Indexes, depending on what ratios are used. Generally, a higher concentration the thinner the base oils (which are used to improve low temperature performance), requires an increase in concentration of VII's to ensure proper viscosity at operating temperature.
**You may notice that in the range of base oils, ALL of them are lower then the target 9.3-12.5 cSt. This is because the operational viscosity of most oils - conventionals and synthetics rely heavily on VII's (between 5-10% of the formula) to reach the necessary targets while maintaining the proper cold temperature performance.

So, if you are comparing two 5W30 oils, and one has a higher VI, then it will maintain it's flow rate better as the temperature changes. This could be due to a higher concentration of VII's, the use of High VI base stocks, or a combination of both. The bottom line is that in cold temperatures the oil with the higher VI will generally be thinner for better flow, and at high temperatures the one with the higher VI will thicker and able to better maintain hydrodynamic lubrication. So if you are concerned with maintaining the right flow rate (viscosity) oil for your engine and your operating conditions, then the VI is the most important oil specification.

A word of caution: be careful trying to predict an oil's viscosity at extremely cold or extremely hot temperatures using the VI as a reference. Because the VI is a calculated number, which uses a somewhat arbitrary formula, you can get widely different results due to the differences in formulas of the finished oils. Most online calculators can give an estimate, but this may vary because the components of the finished oil's formula may be designed to produce a specific result, not just based on the calculated VI.


BUT WAIT, THERE'S MORE!
Unfortunately, an engine isn't always at hydrodynamic lubrication, in fact, a typical engine is operating at various levels of boundary, mixed, and hydrodynamic regimes in multiple points of the engine at the same time. Finished oils are designed not based on viscosity performance alone anymore. (15-20 years ago it was one of the most important considerations) Now engine oils are also about film strength, sacrificial additive layers, detergency, and other important characteristics. In many cases where thinner oils are being used - to improve fuel economy - oils are coming with better concentrations of additives to prevent wear in boundary and mixed lubrication regimes. Bearing Materials are also being improved to prevent wear in boundary and mixed lubrication conditions. Unfortunately there is no standardized data which appears on the oil's technical data sheet which can tell us about the wear protection. It's not like the oil manufacturers publish the results of the engine sequence tests. Without this information it is impossible to quantify the difference in any oil's ability to protect an engine. On top of that, operating conditions have lot to due with how an engine oil/engine combination performs, so without extensive UOA and other testing comparison is really just a guessing game.

So why is VI the holy grail of oil specification: Matching the right viscosity properties of an oil to your engine is one of the important considerations in selecting the best oil for your vehicle. It will continue to be very important when the next round of oil specifications are completed, as new engines will be designed for thinner oils, and the new thinner oils (starting in 2016) may not work for many older engines. As far as comparing the viscosity performance of two oils goes, the VI is king - as long as you remember that the VI comes from a combination of baseoil selection and additive formulation.

Why is the VI not the holy grail? there is much more to an oil's performance in an engine than just the viscosity performance. But most of the information you need to compare additive package performance isn't available on your typical specification sheet, mostly because there aren't bench tests that can be used to accurately quantify that information. So through some experimentation and work on your own vehicle (if you care and have the time to do so), you can find a great oil/vehicle combination that suits your needs, and it may not necessarily need to be the one with the highest VI.




Great explanation. Thanks

 

[martinq]

Solarent, that is one meaty post. Thank you!

 

[KrisZ]

Originally Posted By: Solarent
The bottom line is that in cold temperatures the oil with the higher VI will generally be thinner for better flow, and at high temperatures the one with the higher VI will thicker and able to better maintain hydrodynamic lubrication.


Very good post.

But there is one thing that you missed in the quate above. Yes, at 100C and above the higher VI oil will resist thinning better, but at any temperature below 100C that oil will generally be thinner than a lower VI oil and therefore may not provide adequate hydrodynamic lubrication in an engine that operates below 100C mark and was designed to operate with lower VI oils.

Too bad that there are no graphs (that I'm aware of) showing oil viscosity running across temperature range, not just two points. That I think would be a very interesting graph to analyze, even if it was for one oil.

 

[martinq]

Originally Posted By: KrisZ
Yes, at 100C and above the higher VI oil will resist thinning better, but at any temperature below 100C that oil will generally be thinner than a lower VI oil and therefore may not provide adequate hydrodynamic lubrication in an engine that operates below 100C mark and was designed to operate with lower VI oils.


You need to select an oil that has the right viscosity for the operating temperature, simple as that. A higher VI will benefit in many ways; by reducing wear at lower temperatures, less heating due to pumping losses, etc.