YS recommends 15 to 20% nitro. They run great on 20%. Too little nitro and your throttle response is poor and the power is way down.

I run Powermaster 20/20 in mine. 20%oil 20%nitro. The oil residue out the exhaust is clear and clean. They must use some high quality oil! By the way it is all synthetic. A pattern flier told me to stay away from castor. I do believe that 2% will not hurt anything though.