Shearing and Oxidation

[marco246]

I've become interested in how motor oil viscosity differs from "new" under stress of usage and time. I'm pretty ignorant and am searching for enlightenment here. (No "Grasshopper" jokes, please).

That oils will fall out of grade because of shearing is pretty clear. My own experience with Mobil 1 0W-40 shows that an old Ferrari V-8 will shear this stuff down from 14+ cSt at 100*C to 11.3 cSt in only 1200 miles. The good news is that the shearing appeared to have no effect at all upon the protection provided by this oil to this engine. I have heard that over time, however, the oil will thicken back up because of oxidation, and the implication is that this is a good thing. Why would this be good?

More fundamentally, what is happening in oxidation and why does it cause thickening? I assume (and it's been 40 years since chemistry class) that oxygen is bonding with elements in the oil and forming new or different molecules.

Will an oil that is becoming oxidized lubricate more poorly than one that is not? Does it matter? If an oil thickens from a 40 to a 50 weight because of oxidation, is that particularly meaningful other than representing probably a reduction in fuel usage?

Well, I feel better getting all that off my chest. If I can get some answers that even I can understand, I'll start working on the Meaning of Life.

Cheers, Mark

 

[bruce381]

""Will an oil that is becoming oxidized lubricate more poorly than one that is not""

Some testing has shown oil that is getting oil lubes very well the biggest problem is that oxidation of oil leads to sludge and varnish which will clog and engines oil system and lead to hot runing and emmisions system problems.

SO any thickening due to oxidation is NOT a good thing even if if brings the vis back up to "normal".

Change it out before that occurs
bruce

 

[buster]

I never liked that about Amsoil. That is the main reason I never extended my drains with it, or any oil. Amsoil in particular (SL stuff) did not hold it's viscosity very well at all. I find it ironic how the API SM spec made Amsoil a better oil than the previous SL stuff.

 

[gtschulz]

buster you lost me with your comment about Amsoil. Could you please explain what you mean. I have extended my ODIs (12 to 15k) for years and have not experienced any problems. Am I missing something?

 

[jorton]

Quote:

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"SO any thickening due to oxidation is NOT a good thing even if if brings the vis back up to "normal".

Change it out before that occurs
bruce

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If the OP does not get an UOA, what time frame or miles on oil change should he change oil?

 

[Pablo]

Buster is going by assumption, falsehoods, and frankly ignorance. Sure some older formulas of Amsoil might drift up from 12 to 13 cSt, but to say that was caused by oxidation is simply incorrect. It gives the impression that the oil has suddenly turned into a gelled mass that won't flow and to say such things is crazy talk, IMHO. The main reason I know this slight thickening was not oxidation was by comparing ooxid. numbers to virgin samples AND by looking at the wear numbers.

To answer the OP's question - oxidation can and does cause oil to break down in a variety of ways. It does cause undesirable molecular changes, linkages, gelling, some shearing and free radicals - none of these are a good thing and can cause certain wear spikes.

 

[RI_RS4]

Bruce, could you clarify "Some testing has shown oil that is getting oil lubes very well ...", I think you meant to say "Some testing has shown oil that is oxidized lubes very well ..."

You go on to say that oxidation leads to sludge and varnish which is bad, and then conclude that "any thickening due to oxidation is not a good thing."

I guess that's a true statement. But it could lead to removing the oil prematurely. One could also say that any wear is not a good thing. Could you clarify your philosophy on when oil should be removed from an engine?

 

[JAG]

Quote:

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That oils will fall out of grade because of shearing is pretty clear. My own experience with Mobil 1 0W-40 shows that an old Ferrari V-8 will shear this stuff down from 14+ cSt at 100*C to 11.3 cSt in only 1200 miles. The good news is that the shearing appeared to have no effect at all upon the protection provided by this oil to this engine. I have heard that over time, however, the oil will thicken back up because of oxidation, and the implication is that this is a good thing. Why would this be good?

---

It's certainly not good. The level of badness will have to be decided based on the entire UOA data set. Years ago I did see several cases of this oil doing that on long enough OCI's. But in more recent times, I have not seen the thickening back up. That may be due to the formulation change it's had or maybe the used oil analysis that would show it have not been posted...I suspect the former.

I've been very interested in oxidation too so I found some articles and have posted some of them on this forum..they are there for the searching. Oxidation is not the only degradation mechanism that occurs and it's all very neat stuff but evil on engines. Despite my attempts to avoid it from day 1 of owning the car, I still ended up varnishing/sludging up the PCV system in my VW GTI 1.8T and the replacement of the parts cost me over $500. Clearly I am going to have to do things differently this time around to avoid it happening again.