Any oil spec'd for wet clutches in ATV's and motorcycles. 10w-40, unless you live in a super cold climate, and then 5w-30 is what mine calls for and what most people use as far as I know, but frankly as long as you have oil in it then you're ahead of the game. 
I use honda oil, but you don't have to.
I use honda oil, but you don't have to.