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I could probably get away with a lighter viscosity (5) in the winter in colder temps, but I wouldn't recommend it during summer months on a built motor. It would actually probably quiet down the motor a bit on cold starts, as well.
My father rebuilt catapillar diesel engines way back in the day, and I remember changing his oil and him telling me that I should always run a 10w40 in the summer and a 10w30 in the winter. Guess it's just one of those things that stuck with me?
I run 5w30 or 5w40 in winter months, after that it is a 10w30, 5w40, or 20w50. This is speaking about a wide variety of vehicles I see everyday, not just SR20's