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.
While I agree that they run better and seal better with castor, I don't buy that theory. Mostly because I have run with as little as 30% castor 70% synthetic with no problems except for hot restarts which started on the first tank and lasted to the last tank. Back to 100% castor and no problems. The thing is that castor oil is thicker and doesn't vaporize. Has little to do with varnish.
The only reason to use so much oil in C/L is to improve the 4-2-4 break, other than that you can use the same percentages as R/C. One exception is the Fox Stunt. If you look at it you see it doesn't have much in the way of cooling fins. Many other early engines were also this way. Reduce the oil and it will get hot even when 4 cycling, especially with a cowled stunter.