i have always used morgan omega 15%. this is technically 2 stroke fuel (castor/synthetic blend) i used this fuel for everything with the exception of my heli. i am about to buy a gallon of omega 4 cycle mix, still castor syn blend but i believe it has more oil. this is the only fuel my LHS has right now and i figured the little extra oil couldnt hurt especially on break in. just read the running-in procedure for your o.s. 91, my engine is turning a 14-7 Graupner 2 blade at 8450rpm at optimum setting on the first tank of fuel! this engine is really sweet. i then turned the high speed needle 3/4 - 1 turn back to 6900 rpm for most of the first 2 tank fulls. i like using a little castor all the time just for the peace of mind but if you set your engine properly then synthetic should be fine. its just that there is a lot more parts in a four stroke and the castor helps. just my .02