Whats better for a 91fx a caster mix or fully syn like coolpower? I really like some castor in my .61fx abc but with the 91 having a ringed piston would the castor be needed? I dont want a gumed up ring on the 91 and have heard that castor is not needed with ringed engines. Also any body like the Byrun fuels as that is what my local hobby shop carries other than the coolpower?

Ive evan thought about mixing the two, any comments on this?