If you use a good grade of castor and tune your engine correctly the only "varnish" you will see is a little brownish baked on castor on the head and silencer. And that depends on how well the mating face of your silencer and exhaust stack seal.

Varnish in the engine using castor oil as the lubricant is a product of:

1. Running too lean and overheating

2. Using a poor grade of castor to begin with

I use castor as 50% of the lube package (special degummed 1st pressing) and have never seen varnish inside an engine of mine.

No synthetic has the film strength of castor. But let's not start another oil thread, please!