ControlLine,
All Fox .15's from the first "steel fins" to the present schnuerle are iron/steel. Most Fox engines are iron/steel.
The reason for using all castor in a used iron/steel engine, especially one with extensive running time, is that the castor forms a varnish in the porous iron piston that supplies some of the compression. If you add a little synthetic to your fuel (as much as 3% I understand) it will remove the varnish and "clean" your engine. When you have this happen you may be surprised at the compression your engine doesn't have.
Not to worry. By switching to all castor, most if not all of the seal will come back as you run the engine. You may experience harder starting (low compression) while it is building up.
The caution about "RC" fuel is twofold. First is the synthetic issue. The other is the AMOUNT of lube. Most use lube amounts adequate for ABC, BB engines. Engines without BBs and engines that are iron/steel need more. About 22%-25% is recommended for most. The Fox .35 needs 25%-29%.
Additionally, it seems (opinion) that CL use requires more lube than RC. Perhaps it is the stresses imposed by constant changing loads during maneuvers.

George