Amsoil offers two ATV oils: 5w-50 Synthetic ATV/UTV Engine Oil, and 0w40 Formula 4-Stroke Power Sports Synthetic Motor Oil. My manual calls for 5w50, and that oil is marketed to Polaris owners. However, an Amsoil dealer suggested that the 0w40 would provide superior performance. Apparently, the 5w50 was just made for Polaris owners who insist on that viscosity, but the 0w40 had a superior additive package and better performance characteristics. Can anyone offer further insight?