I burned a gallon of Duke's fuel, 5% Nitro, 29% oil, all castor in a OS .40 FP and had absolutely no problems. Good reliable idle and the power one would expect from 5% nitro at full throttle. The FP line of engines were very similar to the LA line of engines except they had the needle valve on the carburetor instead of at the rear of the engine. I also burned many gallons of ducted fan formula, 24% oil with no problems in the same engine. The biggest drawback to the high oil formulas is the fact that oil is expensive and raises the cost of fuel.