This is my experience with castor in a ringed engine. I ran an Enya 45 with twin rings on a dose of 25% all castor for all it's life. It had quite heavy use because it was in a competition CL stunter so there was much flying done with it. Every year or so I'd strip it down, clean out the carbon build up and put it back together for another year or so. I never encountered sticky rings, and it was only after maybe a few hundred hours that it began to be harder to start (which in my case meant maybe 5 or so flicks instead of one). A new set of rings and it had a second life.

Lately I've been using an ST G51 with a single ring but still on 25% all castor. It's only done about 30 hours so far but no signs of a sticky ring. Word has it that even a relatively small amount (maybe 10%?) of synthetic in the total oil quantity will keep the engine clean inside (the carbon build up).