I know the owners manual recommends the use of 5w-20 for our beasts. Thats what I have always used. Recently, I was told, that I should be using an oil with with a higher viscosity irregardless of what the manual says. This particular mechanic recommended at least a 10w-30, but that 10w-40 would be the best for engine life. Is this true? Anyone out there using oil of this viscosity?