Read some back posts on this issue. Manual recommends multi viscosity ie 10-30 if I remember correctly. Many good choices including ATF, but many modern engine oils have friction modifiers included which are good for modern car engines, but not so good for your clutch which runs in this bath. Remember this is a 2 stroke engine which gets its "motor" lube from the oil in the gas. The oil we put in the KDX is only for the gears and transmission. A full quart makes the whole machine quieter. Cheers John