Oil resistance to flow at operating temp question

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superchargedx

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Forgive me if this is a rookie question,

As I read the motor oil university page I feel a tad confused...
I understand that a 5w30 conventional oil is essentially a 5w oil that is modified by additives to flow like a 30w at operating temp..
So that being said, if oil (Weight) is the resistance of oils ability to flow... why does it become "thinner" and flow at a faster rate when hot???

I am a Mazda Tech and have a lot of automotive experience but I am having a hard time getting my head around this one!!
LOL, once someone can sum this one up for me I will hit you with my synthetic questions!!

Sean
 
When you put cooking oil on a pan, and turn on the heat. Do you notice the the Cooking oil becomes less viscose?

Same with Engine oil.
 
Understood but,

Isn't the theory that as oil temp rises, it starts to act as a 30w oil (In the 5w30 example).... this would mean that it now has a higher resistance to flow and is now "thicker" moving at a slower rate through journals/bearings..?.. So why does oil drained from a hot engine have such a thin/watery consistency ?
 
All oil is thicker when cold and thin when hot. The W30 is not 30W at operating temp it is around a 10W when hot. 5W30 is 5W that is manipulated to behave like a straight SAE 30 would behave like at operating temperature while giving a thinner oil at start up when most wear occurs. Straight 5W would be almost as thick as water without being modified.

Oil Type Thickness at 75° F Thickness at 212° F
Straight 30-------250--------------------------10
10W-30------------100--------------------------10
0W-30--------------40--------------------------10
Straight 10--------30--------------------------6
 
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Originally Posted By: superchargedx
Understood but,

Isn't the theory that as oil temp rises, it starts to act as a 30w oil (In the 5w30 example).... this would mean that it now has a higher resistance to flow and is now "thicker" moving at a slower rate through journals/bearings..?.. So why does oil drained from a hot engine have such a thin/watery consistency ?


act like a 30 at that higher temperature. While a straight 5 would always be thinner than a straight 30 at the same temperature, a 30 at room temperature is much thicker than a fully warmed up 5.
 
to answer this question, you first have to learn to understand viscosity improvers and polymers added to make multiviscosity oil.

To control the viscosity ("thickness" or "ease of flow") , temperature-sensitive polymers called "viscosity imodifiers" are added into the base oil.

When during low temp: the polymers within the oil will curl up to "relax" the oil flow; during high temp: the polymers will "stretch" to "restrict" oil flow.

That's why multi-visc oils defy conventional wisdom RE: hot oil goes "thinner" and should flows "freer" or "Better" but with viscosity improvers, it's the opposite way around.


(*studied this back in auto mech certification days so my memory is a bit rusty*)

also ref: http://www.machinerylubrication.com/Read/1327/viscosity-index-improvers

Q.
 
Viscosity is the resistance of oil to flow "At Any Temprature," but the oil will have different resistances to flow at different temperatures.

Here is a synthetic 5W30 spec. Notice different Kinematic viscosities at 40C and 100C:

API Service Class SN/SM/SL/SG/CF
Viscosity Grade SAE 5W30
Vis @ 100°C, cSt 10.6
Vis @ 40°C, cSt 62
Viscosity Index 162
CCS Viscosity, Poise, @*C 60@-30
Pour Point, °C -45
Pour Point, °F -49
Flash Point, °C 252
Flash Point, °F 486
NOACK Evaporation Loss,1hr @ 482°F (250°C), % 6
HTHS Vis, cP @150°C, ASTM D4741 3.8
 
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Oil Type Thickness at 75° F Thickness at 212° F
Straight 30--------250----------------------------------10
10W-30-------------100----------------------------------10
0W-30--------------40-----------------------------------10
Straight 10--------30-----------------------------------6
Straight 5---------20-----------------------------------4
Straight 2---------15-----------------------------------3
Straight 0---------12-----------------------------------3 ( est )
 
I really appreciate these answers,

Quest..... So what you are telling me is don't judge the viscosity based on the appearance of oil thickness when draining from a hot or cold engine.... it is the modifiers that are controlling viscosity during operation regardless of the physical thickness that it comes out of the pan??
 
Originally Posted By: superchargedx
I really appreciate these answers,

Quest..... So what you are telling me is don't judge the viscosity based on the appearance of oil thickness when draining from a hot or cold engine.... it is the modifiers that are controlling viscosity during operation regardless of the physical thickness that it comes out of the pan??


Yes, that's right. Without the viscosity improvers viscosity would be too light to when it's hot.

And without the pour point depressant viscosity would be too thick when its cool, or cold.
 
Quote:
Group V (5) based synthetics are usually not compatible with petroleum or petroleum fuels and have poor seal swell. These are used for air compressors, hydraulics, etc. It's the Group IV (4) PAO based synthetics that make the best motor oils. They are compatible with petroleum based oils and fuels plus they have better seal swell than petroleum.


He got that wrong!
 
My mind sometimes can't grasp these things all so well, but I was under the impression that use of esters, by virtue of having more possible configurations, should make it possible to formulate an oil that more closely matches the expected needs of a particular machine but can be more expensive due to the complexities of producing them. Is that so, MoleKule?
 
super c -
A straight weight 5 will be thinner than a straight weight 30 at any and all temperatures.
A 5-30 ACTS like a 5 cold, and a 30 hot.
But the thing is: a 5 cold is thicker than a 30 hot.

That's all there is to it!
 
LOL,

I think i get it but wow, still a bit confusing.....

mechtech- when you say that "a 5 cold is thicker than a 30 hot", you mean in a physical "Pour" manner and not the actual resistance to flow "viscosity"..... correct?

Sorry guys, just want to get a good grip on the basic fundamentals before further questions/learning haha!!
 
Let me give this a shot. If you put straight SAE 30 in your engine, and drained it out when it was hot, it would be just as thin as that 5w-30 you're draining out hot.

So 5w-30 oil is basically a 5-weight oil that doesn't thin out as much as a straight 5-weight oil would, because of the viscosity improvers that are added to it. It does get thinner as it heats up, but thins out much more slowly than a straight-grade oil would.
 
Think that I have got this down for the most part!!

Secondary question,
I often see people referring to different brand (lets say 5w30) as being thicker or thinner?? How do two oils of the same grade but different brand have different viscosity? This seems crazy unless it is so trivial that it means nothing and people are looking for something to pic on....??
 
Originally Posted By: superchargedx
Think that I have got this down for the most part!!

Secondary question,
I often see people referring to different brand (lets say 5w30) as being thicker or thinner?? How do two oils of the same grade but different brand have different viscosity? This seems crazy unless it is so trivial that it means nothing and people are looking for something to pic on....??

yes arguing over a maybe 2.5cSt difference between the average 20W & 30W is third grader, but no one is going to admit they are wrong, self included...

With a few exceptions, there is often more variance within the 30W grade than between the average 20 & 30W oils...

You apparently didn't check the link I provided yesterday in page one of this thread...

http://www.upmpg.com/tech_articles/motoroil_viscosity/
 
Because the SAE viscosity grades represent a range, not a single discreet value.
If you think about it, it would be impossible for every thirty grade oil to have exactly the same kinemetic viscosity at any given temperature.
There must be some variation across blenders or even any given brand.
Therefore, kinematic viscosities for any given SAE grade are specified as a range of values.
Some blenders produce oils toward the thinner end of the range, while some blend toward the thick end.
An additional factor is HTHS, which can make an oil that meets the SAE kienmatic viscosity range for a thirty actually quite thick at higher temperatures.
It also isn't all viscosity index improvers.
All basestocks have a natural viscosity index, and it is possible to blend a multi-grade oil with no viscosity index improvers at all, if a quality basestock is selected.
 
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