RE: Changes?
Lunbom,
Depends on the engine.
The Fox 35 control line (lapped iron) engine runs the best on 5% nitro, 25% to 29% all castor fuel. The other early design lapped iron, bushing (non ball bearing) control line engines run best on the all castor fuels, at least at first, then can be run on a 50/50 blend, still of high total oil (20-23% total oil).
The modern ABC and Ringed ball bearing Fox engines run just fine on nearly any 5% to 10% nitro, castor / synthetic blend fuel.
I have numerous Fox engines, from the older ringed big frame 40's to the current 74's. I run all of them on whatever 10% or 15% fuel I have extra laying around. The Ringed engines run just fine on all synthetic fuels as long as you have enough oil, the ABC engines will like a castor/ synthetic blend.
Hope this helps.