Bulldozer said: (quoted from post at 12:46:16 04/09/13) Multi weight engine oil has viscosity index improvers that reduce the thinning with increase in operating temperature.
An SAE 10-30 wt oil is base 10 wt with viscosity index (VI) improver (heat sensitive additives)
The VI improver additives are susceptable to shearing damage in non synthetic oil. The damage can occur very early in the life of the oil, reducing the effective weight of the oil from 10W-30 to 10W-20.
Ever notice that when the oil is changed when using non synthetic multiple weight oil, the oil pressure gauge shows an increase in pressure? After the oil is used for a while, the oil pressure gauge shows a reduction in the oil pressure because the VI improvers have been damaged by internal engine shearing action.
Using synthetic muti weight oil the shear damage issue is basically eliminated.
Still, Prefer using straight SAE 30 non synthetic and change it once a year in my N whether it is used or not.
All oil shears down. I don't know where you get your empirical data but I prefer laboratory analysis to anecdotal observation. Excessive shear loss on a quality conventional multi-grade oil is darn near non-existent. A used oil analysis is a good way to see this and there are many consumer testing services that will provide you with one. Just send them a small used oil sample and for ~$20 they will give you an analysis similar to this one. This UOA is a typical result showing the performance of a modern semi-synthetic/conventional 5W30 and as you can see the oil is still well withing viscosity grade after 5K miles and could probably go another 5K if you wanted to push it. The typical failure mode of a modern oil is over-thickening due to oxidation not viscosity shear down There are hundreds of similar UOA's available for viewing on the BobIsTheOilGuy website and you will find this level of perfortmance is typical of conventional oils with synthetics doing much better.
Conventional SAE 10W30 can (and occasionally is) formulated using a very high viscosity index (VHVI) conventional base oil and no viscosity modifiers whatsoever. SAE 10W30 synthetics are commonly formulated without viscosity modifiers. So this idea that the W grade is the "real" viscosity is factually inaccurate and only serves to confuse/scare people. Buy a good oil and change it on a normal interval and you won't have a problem with shear down.
TOH
PS> I added a second UOA for a conventional SAE 5W30 - still going strong but just barely out of grade after 9.2K miles - nearly double the recommended OCI for a conventional oil. Note the lab says he could stretch the OCI to 11K ....
This post was edited by TheOldHokie at 11:59:06 04/09/13 3 times.
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