I'm hooked on Amsoil 10-W-40, been riding for over 31 years, everything from old husky's and yamahas in the dirt and over 8 Harley Davidsons, currently I own a 2008 YFZ 450, 2009 YZ450f and a 2009 Screamin Eagle Springer softail. I've switched all of them over and the most noticeable was the difference in engine noise in the YfZ 450 and Springer, made them sound totally different nowhere near the top end noise. Check your
service manual for the viscosity for your temperature in your area and go for it!!