Quote:
Originally Posted by atomicalex
I gotta write a book but I need a real keyboard.
It's the confusing terminology. Basically, teh 0, 5, 20, etc are actual viscosity specs, but they are kind of inside out. It may be monday before I get to my laptop, but I will be back to elaborate.
Without an actual viscometer in hand, and without controlling for the nature of degradation that occurs, the "clean" or unused viscosity is a guide. The difference between 35 and 40 cSt may not be that much after a few hours of running.
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I simply compared all the clean oil viscosity values of 0W20 oils. I don't have any interests in oils outside of 0W20 so I can stay per manual's specs, especially since my driving is at most a spirited canyon drive.
Yes clean/unused/virgin oil viscosity is merely a guide, but absolutely a great place to start (especially since that's all I have to work with and I'm too lazy to collect and compare used oils while standardizing for room conditions; full synthetic oils are also less prone to degradation). And yes, the temperatures aren't going to remain at 40C. But the rough starts are my primary concern because 1) all the 100C viscosity values of 0W20 oils are all about 8 cSt (thus similar viscosity at normal operating temps), 2) majority of engine wear is during the cold starts (or so I have read in various places), and 3) I'm making a general assumption that any good brand oil will provide sufficient engine protection at ideal operating/warmed up temperatures.
I also feel the difference for a 40C 35 cSt vs 45 cSt is quite significant - 28.6% greater (45/35). You should also consider that 40C is still very hot (104F).
Let say its 50F outside (10C), which isn't that cold. Chart the following viscosities: Idemitsu Zepro 0W20 40C - 34.9 cSt, 100c - 7.9 cSt vs Mobil1 0W20 40C - 44.8 cSt, 100c - 8.7 cSt.
Use this (or any other graphing calculator of your choice):
https://www.widman.biz/English/Calculators/Graph.html
At 50F/10C, you're looking at a rough start of 120 cSt vs 180 cSt. That is 50% more viscosity at real morning temps (180/120). Of course, this is gap is even more severe if you live somewhere even colder. Say its 0C (32F) outside, you're looking at 200 cSt vs 320 cSt (60% - 320/200).
Now I know these graphs are theoretical calculations, but accurate enough to provide realistic expectations .. probably <5% deviation.
For someone like me, who does lots of short distance driving, I also want to provide the least strain on my engine when its cold. And my (edit: by*) least strain, I consider it to be the oil that provides the viscosity closest to full operating temperatures of ~8 cSt (based on manufacturer's suggested 0W20 and the average 0W20 oil viscosity at 100C of ~8 cSt).