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Pretty sure there are diminishing returns with going thinner right? On ft86club.com people say that 0w-20 is so thin that at track temperatures of like 260F none of the oil is being bypassed anymore as the pressure is only in the 50psi range at 7400rpm.
We're talking 8.7 vs. 8.2 cSt. I never read up on how units work but that can't be a big difference. If it drops your oil pressure 1psi, that might get you like 0.1% more mpg. A 30 weight oil is around 10-11 cSt at 100C, and the fuel economy gain from going from 30 to 20 is very hard to measure.
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