Thickening and Shearing

[Craig in Canada]

In the time I've spent here I've noticed that as people use their oil, some thickens as it wears and some thins due to shearing. Mine thinned from 14.x cSt to 12.5 cSt.

I assume all oils shear somewhat and it's a matter of which effect wins. I understand that larger viscosity spread makes shearing more likely, but I see GC 0W30 accused of thickening.

Why do some thicken and some thin? Is it the engine, or the oil, or a combination? Personally, I'm only interested in full synthetics for the purpose of any discussion but others might find dino-related info interesting.

Craig.

 

[TooSlick]

Thinning is generally due to fuel dilution and polymer shearing. Thickening is due to evaporation of the lighter fractions of the basestock and add pack and from oxidation/nitration. The latter result in the formation of insoluble polymers.

TS

 

[mechtech]

It seems that the normal progression is shearing to a lighter viscosity, then evaporating to a thicker viscosity.

 

[Titan]

Thickening before shearing is the goal of a sheep-herder...they get more wool that way.

Oils like GC, which have very little (if ANY) VII's, don't have this reason to shear, so, they don't.

 

[GMorg]

I'll throw-in soot loading as another potential thickener.

 

[Craig in Canada]

So you're saying that during one extended OCI you can see both effects as the different components of the oil deteriorate?

 

[GMorg]

Absolutely

 

[Drivebelt]

There is some data about PAO’s thickening in this article …

Next Generation PAOs.pdf

For the 0w30 synthetic PAO oils, depending on the basestocks used, some had VII's, some didn’t.

In the HOOT (hot oil oxidation test) of 5w30’s, one of the blends thinned 9% at 144hrs, and then thickened considerably (59.4%) at 177hrs. The other 5w30 blend (different base oils) had almost no thinning/thickening at 144hrs, but then thickened quite a bit by 177 hrs. This was at 165C -- quite a bit hotter than one would expect in an engine environment though.

There was a couple of engine tests, but they look like they were all done on diesels And soot is a factor in oil viscosity increases in diesels.

 

[Louie's gone fishing]

Does Mobil 1 0w-40 shear in high performance engines?

 

[Blake Sobiloff]

Yes, and then it begins to thicken back up--plenty of used oil analysis and discussion around this phenomena--"search" is your friend.

 

[Bill]

quote:

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Oils like GC, which have very little (if ANY) VII's, don't have this reason to shear, so, they don't.

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Oils with little or no VII may not Shear,
VII (Viscosity Index Improvers) are seen as a weak link in a oil that may break down during a OCI,
any oil, synthetic and conventional will thin out because of fuel dilution.

GC appears to thicken at around 10,000 during a OCI in some vehicles.


The only oil I have seen thin out and then thicken up is Mobil 1 0W-40.

Are there any other oils that have this trend of thinning out and thickening up again ?