I was talking about 5W20, 20 grade oil are thinner than 5W30. The # is determined by some geeks at SAE, I don't make it up. I wouldn't use it on the 370 unless you really want thin oil and try to save a drop of gas. RX-8 went to a 5w20 recommendation in the states in order to get out of the gas guzzler bracket. There is a reason why most people lose their engine before 100K while my old 90 went 220K on the original engine.
Synthetic oil can handle 300F before it starts to break down, that is a fact. The question is, how thin would the oil get at that point?
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