18K RPM on a .91 FX would not be a reasonable RPM level. That's much too high, and well out of a reasonable RPM range for flying almost any airplane. We really doubt the engine will hold up very long if you're going to be running it at that RPM. At high RPM levels, you'll also have to go to a fuel with 22%-24% oil content to make sure the lower-end of the connecting rod gets enough lubrication...otherwise, it can seize and blow in seconds. We'd blown rods on fuel with 15% nitro and 18% oil content in ducted-fan engines...they'll run beautifully on 10% nitro and 24% oil. Oil should be a blend of castor and synthetic, preferably 50/50...more castor is better.