it is an urban legend now, it was true back in the day when synthetic oils didnt have ester content.
for thinking 0W40 is too thin compared to 10W40, think again! the 0 part determines its cold (and i mean cold) weather performance, and the lower the number the better. the number after the W is for its operating viscosity. having the difference between the numbers too high generally means there are lots of viscosity index improvers which shear out over time, however M1 0W40 seems to have exactly the same viscosity index as castrol syntec 0W30 from germany, which doesnt even contain viscosity index improvers! i would assume that the M1 does however, but not much.
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