I use 10w40 conventional oil. I just changed oil in the Polaris 700 today and used 10w40 Castrol Start-up, I bought on clearance. Polaris four stroke engines would probably be the least fussy four stroke atv, you will find. They have no wet clutch in there. They are basically just four stroke motors I have used 5w40, 15w40, 15w50 and 10w40 without any issues or differences. I wouldn't hesitate to use 10w30 either. Ive used 10w30 in generators, lawnmowers and Honda ATCs. I understand, they cost so much, so your concerned about what oil to use. One thing is for sure Polaris oil is a rip-off.