15% nitro should be fine but you ABSOLUTELY need castor in the Cox engines. You might get away with a 50/50 mix of castor and synthetic at 20% total oil content but you'd be safer with all castor. If you run it with the Omega then you'd better have a ball joint reset tool handy

These engines use a ball joint to connect the rod to the piston which allows the piston to rotate and castor is the only oil that will stay in place under these extreme conditions.