Motor Oil University Chapter 2

[paulri]

I was reading this Motor Oil University article --towards the bottom of the article, and if I understand it correctly, the first number (the w number) in the viscosity grade not only tells you how cold it can be and still have the engine start up, but it also controls or influences how viscous the oil is after startup. Is this actually the case? I always thought that the only thing that the first w number affected, was how cold it could be, and still be able to circulate through the engine; but that after starting the engine, the oil behaved as the second grade number.

Another way of saying this is—let's just assume 2 oils, that are alike in every way, except for the pour point depressants required to make one oil an 0w, with the other a 10w oil. So we have the two oils, an 0w-30 and a 10w-30. Once I start the car, will the 0w oil flow any quicker at cold temperatures? Or does the 30 viscosity grade kick in at that point, and both oils would flow at the same exact rate?

Going back to the article, if I'm understanding it correctly, it was saying that the oil with the lower winter grade, will also have a lower viscosity at 75 degrees F. Or have I misunderstood it?

 

[CR94]

Originally Posted by paulri
... if I understand it correctly, the first number (the w number) in the viscosity grade not only tells you how cold it can be and still have the engine start up, but it also controls or influences how viscous the oil is after startup. Is this actually the case? ...
Yes, while the oil is still cold. The number after the hyphen applies after the oil is at operating temperature.

 

[kschachn]

The only real flow that the ASTM tests (which are part of SAE J300) are concerned about are the low shear flow to the oil pump pickup tube (ASTM D 4684). Cold-cranking is addressed by ASTM D 5293 and it doesn't mean your engine will or won't start, it just specifies a target range of apparent viscosity that the oil must achieve to be given a specific winter rating.

So two oils that have an identical winter rating will perform the same (within the range of allowed apparent viscosity). People on here always want to drag "flow" into the mix but it isn't relevant in this context except as noted above. The oil does not "act like" anything, it has an apparent viscosity that meets an assigned range and is labeled as to the performance it achieves.

Winter grade is not directly connected to viscosity any other temperature other than the one at which it is tested.

 

[69GTX]

If the ambient temp supports it, at cold startup a 0w will flow no faster than a 10w. If the pump can pump it, they flow the same.

it will take from several minutes to 20 minutes or so to reach normal operating temps depending on ambient temps and engine load/rpm. During that time your oil is making its way from a 0w or 5w.....to a 30w.

 

[paulri]

I take your comments to mean that in my hypothetical example above, both oils would have the same viscosity once the engine was started--at least, if the only difference was the winter rating, and additives required to get the 0w oil to meet the required pumpability test. I realize that the viscosity grades are a range, and that there can be differences in viscosity among various 30 grade oils.

Originally Posted by kschachn
So two oils that have an identical winter rating will perform the same (within the range of allowed apparent viscosity).....Winter grade is not directly connected to viscosity any other temperature other than the one at which it is tested.

 

[kschachn]

Originally Posted by paulri
I take your comments to mean that in my hypothetical example above, both oils would have the same viscosity once the engine was started--at least, if the only difference was the winter rating, and additives required to get the 0w oil to meet the required pumpability test. I realize that the viscosity grades are a range, and that there can be differences in viscosity among various 30 grade oils.

It's completely dependent on temperature. At some point (below 32F probably) the 0W rated oil will deviate in viscosity from the 10W rated one, but above that temperature the winter rating makes no difference. You use the term "once started", I assume you mean "warmed up to operating temperature". If that is the case then yes, since both are 30-grade oils.

And It isn't just "additives" that affect the winter rating, it is also the base stock.

 

[paulri]

No, when I said "once started," I meant just that. It was clear to me, that the 2 oils in my hypothetical example would have the same viscosity at normal operating temperatures. I was just wondering about the 10-20 minutes in between the engine being started, and arriving at those operating temps.

Living in California, I start the cars when ambient temperature is below 32F, maybe 3-4 times a year, and even then, it is high 20s if not 30-31, if that matters.

 

[ZeeOSix]

Originally Posted by paulri
No, when I said "once started," I meant just that. It was clear to me, that the 2 oils in my hypothetical example would have the same viscosity at normal operating temperatures. I was just wondering about the 10-20 minutes in between the engine being started, and arriving at those operating temps.


All oils have a viscosity vs temperature curve that defines how the viscosity decreases as the temperature increases, and it's not a linear curve.

If a 0W-30 and a 10W-30 oil are both starting at 10 deg F and warming up to 200 deg F, and they have the same viscosity at 200F, then the 10W-30 will start out thicker than the 0W-30 at 10F, and will therefore have more viscosity change between 10F and 200F than the 0W-30 will.

If you can find the KV40 and KV100 viscosity specs for different oils you can plug that info into this calculator and it will draw the viscosity vs temperature curve.

https://www.widman.biz/English/Calculators/Graph.html

 

[dnewton3]

SAE grades are RANGES, not singular points of viscosity.

It's entirely possible that a 10w-40 and 15w-40 could have the (nearly) exact same vis at 100C, or they could be as much as a few cSt apart. The 40 grade is a range; one lube can be at 13 cSt and the other at 15 cSt and still be the same "grade". The same goes for the W-grade statement. A 5W-20 and a 5W-40 can start in the same grade when cold, at their specific rated temps, but that does not assure they will be the "same" vis.

 

[weasley]

The 'W' rating requires the oil to pass three viscosity tests:

- cold cranking simulator (CCS) carried out at -(35-W)°C (where 'W' is the number preceding the W in the SAE grade). This tests how the oil allows a shaft to 'crank' in a close-fitting stator (simulating a main journal in a main bearing)
- mini rotary viscometer (MRV) carried out at -(40-W)°C. This tests the ability of the oil to be pumped at low temperature and as well as having a maximum viscosity (60,000 cP) also requires there to be no detectable yield stress - this is to prevent the situation where a cold oil could pump OK at moderate shear stress but might 'set' at low shear, meaning the oil pickup could suck a crater in the cold oil that the oil doesn't readily in-fill, thereby leading to the pump sucking air. For this test the oil is cooled according to a specific cooling profile, slowly at first then rapidly to the test temperature. This is to try and promote wax formation.
- kinematic viscosity at 100°C. Yes, there is a minimum KV100 limit in the W ratings. These are usually redundant because multigrades need to meet a higher KV100 limit for the hot viscosity grade. However some oils are W-rated only, so you mustn't forget the KV100 limit.

 

[benjy]

dont forget REAL synthetic PAO + Ester oil "curve" differently as they stay thinner in extreme cold + thin less in extreme heat + the better performance lasts longer because oil additives that "prop up" specs when oil is tested as new does NOT last + performance deteriorates as time + miles add up! there are a lot of variables that only the pros really fully understand.