Good question but I'm not an oil expert. But looking at the Recommended viscosity chart in the repair manual, It recommends 40 and 50 weight for high temps and 30 weight for lower temps. From what I understand is that higher weights give better engine protections but lower fuel economy. For my personally, I just switched from 30 to 40, and next time I'll be going with 50 weight.
the first number is the viscosity on cold start, second number is viscosity when the engine is warmed up. best oil to use is probably 5w30 that way when the oil is cold its not too thin and leaks out of your engine. old engines over time accumulate leaks, 5w30 is best
