Have been an Amsoil dealer since the 70's when it was called Amzoil but that is neither here nor there. If you run these aircooled engines hard like I do go with the 20-50 it adds a little extra protection. As stated the 10-40 gives a little better fuel mileage but to me protection vs. fuel mileage is a no brainer. If I can afford the bike I can afford the fuel and we are just splitting hairs anyway.
