For the O.S. Max 1.20 AX engine, you'd normally use a fuel with 5%-10% nitro and at least 18% oil content.

For the non-ringed engines, such as the .46 FX or AX engines, you'd use a fuel with 5%-15% nitro content and 18% oil content. In these engines, though, you'd want the oil mixture to contain castor oil. Non-ringed engines do best when the oil content contains a goodly portion of castor oil.

With nitro content above 20%, you may need to adjust the compression ratio of the engine by adding additional cylinder head gaskets to eliminate detonation. That's common with high-nitro fuels, and may be why your engine "doesn't like" the castor oil...the high-nitro fuel's the problem, not the castor oil.