Lowest octane you can get away with without knocking; I like 91 pump gas. Lower octane fuel ignites faster (the reason it detonates in high compression motors). MS93 smells better and keeps your carb clean, but Rotax motors love pump gas with ethanol. It’s also much easier to tune with pump gas; the ethanol makes your carb less sensitive to weather, so jetting doesn’t need to be as precise. There is a scientific explanation for this, but I don’t know what it is. If you are going to run at a national event where they mandate MS93, plan to go one jet leaner than you would on pump (no oxygen in race gas).