For the OS 91 FX I would use the nitro content fuel that you plan on using for flying. The only thing I would suggest in so far as oil content , is to use at least 18% and have some castor in it because the ringed engines need some for proper ring seating. Wildcat Premin Extra is what I used in mine and it is excellent fuel. One other note is to disassemble the remote needle valve assembly where it attaches to the carb and use a high temp gasket sealer arround all joints. This problem has been discussed thoroughly in RCU and I have found that mine worked great after the fix. Good Luck.