I have been using Amsoil 15w-40 Heavy Duty Diesel and Marine oil in my boat. I change it only once per year and put around 100 hours on it. The engines are 8.1 liter GM that are marinized by Crusader. The engines run at 1,500 rpms half the time and the other half at 3,500 rpms. This boat is heavy at 26,000 lbs. so it is quite a load. Is there a better grade of Amsoil that I should be using? When the oil says Heavy Duty and Marine I am just assuming it means any marine engine, diesel or gas like mine. Any thoughts by you Amsoil gurus?