ORIGINAL: mclintock
Short answer, because instead of burning off during a lean run, it actually thickens in high temps, where synth would just burn off, leaving the metal bare when it needs it most! It's easier to clean off castor varnish than to put metal back on.
Long version:
This is especially true of an iron piston/steel cylinder combo because as temperature rises, the fits get tighter, which causes even more heat until you get "thermal runaway" which can ruin an engine. Castor turning to varnish would cause the engine to slow down to some degree which lessens the extreme heat production. Most synthetics flash off at this point and allow the engine to wear out with metal to metal contact.
ABC engines provide for the cylinder to expand more than the piston which prevents thermal runaway. If the lube reaches its flash point more than normal wear will occur but usually not to such a large degree.
BTW, iron/steel setups allow a varnish buildup in the pores of the iron piston which will compensate for a lot of engine wear. If your engine is showing less compression, sometimes switching to all-castor fuel will buy you some compression as it runs. ABC does not do this because of the high silicon pistons.
Conversely, if your iron/steel engine has a lot of time on it and you switch from all-castor to part synthetic, you may find that your engine is worn out. Switching back to all-castor can restore at least some of the varnish seal.
I put this here 'cause cox engines are more likely to be running with no throttle than a norvel, where if lean one could just throttle down and land.
One of the hottest parts of an engine is the top of the piston. In a Cox engine, directly under that hot spot is the ball socket. With over lean runs and all-synthetic lube you run the chance of the lube flashing off and leaving the ball socket unlubed. For this reason, fuel for Cox engines should contain some Castor oil.
In the other direction, when Cox fuel was all Castor we had to tear the engines down occasionally to remove varnish if we ran them hard. Some synthetic in the fuel prevents or lessens varnishing.
There are some synthetics that are touted to be just as good as Castor. Unfortunately, not all fuels use these types.
Bottom Line: Your engine, your choice.
Sorry if this seems like a lot of rambling. I've never met an engine I didn't like.
George