Help a not-so-noob: shear stability theory.

[Mitch Alsup]

Originally Posted by OilStasher
Hi everyone. Been lurking for years. I already feel like I know most of you and know quite a bit about oil and the site. Feels good to finally make it official.

I have a question about shear stability. Please correct me anywhere if I'm wrong. To my understanding, the smaller the viscosity (number) spread between the winter rating and 'hot' (100°C) rating, the more shear stable the oil is. So in theory a 5W-20 should be far more stable than a 5W-50. Right?


Acceptably correct.

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That said, let's go off topic a bit and talk about the winter viscosity rating of oil. The 5 in 5W-20 is the winter rating. I'm fairly sure that the "5" at a very cold temperature is actually far more viscous than the "20" at 100°C, contrary to what most people think. Oil 'thins' as it gets hotter. I'm assuming I'm correct so far?


The 5W at 40ºF is on the order of 100-150cSt depending on the -xx number. Whereas the -20 has a kv100 of about 8cSt.

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So now my real question:

If the first two statements are true individually, how is it possible that both are true at the same time?


Because it has been to the oil companies benefit to keep the consumer confused using a grade-weight scale instead of edumacating the consumer about kinematic viscosity.

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I would think based on the above statements that a [theoretical] 0W-60 would be the most shear stable oil out there,


whereas it is not very shear stable at all.

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and a 0W-16 would shear like crazy.


which is contrary to your first statement. Which is probably where you went wrong.

If you went in and converted yW-xx into centiStokes at various temperatures, it would all become clear.

 

[OilUzer]

Originally Posted by OilStasher

Ah, this makes a little more sense. So it's not the oil that shears, it's the additives? Hmm. Maybe that's solved. But I think I'm still lost on the "low-spread" (example 5W-20) actually being less of a "spread" than 'high-spread' (example 0W-40). There is less thinning with 0W-40 than 5W-20. Meaning 0W-40 starts moderately thin and doesn't thin much when heated. 5W-20 starts slightly thick and thins a substantial degree down to a 20. Why is 5W-20 less of a spread? That sounds completely backwards to me.

It's amazing. The things I've "known" for years suddenly get questioned and I flip my own world upside-down with technicalities. Haha.


I think I know where the hang-up is ... We are talking about 2 different spreads or delta.

Spread 1:
if you take look at a viscosity curve/graph of an oil, there is a spread or delta viscosity (in cSt) between the readings at 0°C vs. 100°C.
This viscosity delta (or spread) for example will be much larger for a straight SAE30 (30W30) oil than a 10W30 than a 5W30 than a 0W30.

Spread 2:
The other spread you hear people talk about is the xWy (e.g. 0W30) spread. This spread/delta is y minus x (y-x) and has nothing to do with cSt spread that I mentioned earlier.
In this case for example, a 10W30 has a less spread (30-10=20) than a 5W30 (30-5 =25).
In general, the less the spread, the less viscosity index improver (vii) required and therefore a more shear stable oil. I assume we can ignore the base oil qualities, etc. for the sake of this argument ...

When people say the less the spread, the better ... they are talking about spread 2.

Are you a mathematician or an engineer by any chance?

 

[CR94]

Or maybe the confusion stems from an alternate interpretation of "stability." If oil A has better shear stability than oil Z, that does not mean its viscosity varies less between low and high temperature after shearing. Instead, it means A, when at 100°C, after shearing, retains a greater percentage of the viscosity it originally had when at the same temperature---at least as I understand.

 

[4WD]

Originally Posted by OilUzer
Originally Posted by OilStasher

Ah, this makes a little more sense. So it's not the oil that shears, it's the additives? Hmm. Maybe that's solved. But I think I'm still lost on the "low-spread" (example 5W-20) actually being less of a "spread" than 'high-spread' (example 0W-40). There is less thinning with 0W-40 than 5W-20. Meaning 0W-40 starts moderately thin and doesn't thin much when heated. 5W-20 starts slightly thick and thins a substantial degree down to a 20. Why is 5W-20 less of a spread? That sounds completely backwards to me.

It's amazing. The things I've "known" for years suddenly get questioned and I flip my own world upside-down with technicalities. Haha.


I think I know where the hang-up is ... We are talking about 2 different spreads or delta.

Spread 1:
if you take look at a viscosity curve/graph of an oil, there is a spread or delta viscosity (in cSt) between the readings at 0°C vs. 100°C.
This viscosity delta (or spread) for example will be much larger for a straight SAE30 (30W30) oil than a 10W30 than a 5W30 than a 0W30.

Spread 2:
The other spread you hear people talk about is the xWy (e.g. 0W30) spread. This spread/delta is y minus x (y-x) and has nothing to do with cSt spread that I mentioned earlier.
In this case for example, a 10W30 has a less spread (30-10=20) than a 5W30 (30-5 =25).
In general, the less the spread, the less viscosity index improver (vii) required and therefore a more shear stable oil. I assume we can ignore the base oil qualities, etc. for the sake of this argument ...

When people say the less the spread, the better ... they are talking about spread 2.

Are you a mathematician or an engineer by any chance?

Bottle of ST SAE30 bought today … SN plus ?