Re: OIL viscosity 04, F150 4-stroke, 10w30 or 10w40 ??
I'm on the Alabama gulf coast and the dealer i worked for several years(layed off) used 10w30 in everything except the f350.I never saw an oil related problem. I don't think that the differance from 30-40 is a problem with most outboards, due to the unlimited supply of cool water and no trans cooler in the system like an automotive application. Take your pick, but i would use yamalube.