You need to know what fuel you are using- the composition. In my observation, 46-55 2c sized engines that are mounted vertically in trainers run fine with plain castor based fuel. Engines that are mounted inverted or in cowls or larger engines or 4c engines dont do too well on plain castor (just my observation, others may have different experiences) and need a synthetic based fuel.

Almost everyone will agree on this -Dont use fuel that contains no castor. Like I said, castor has a higher flash point than synthetic additives. So when the synthetic additives and methanol in fuel burn up (by chance if you run your engine too lean or it runs at a high temperature), castor is still busy forming the oily long-chain polymeric compounds and does not burn. Hence it provides greater protection to the engine. On the other hand, synthetic fuels burn much cleaner and will not give you that staining (typically called gunking). Best to use a fuel (or fuel containing lubricating oil) which contains some castor. I use Klotz 100 which contains 20-80 mix of castor & synthetic in the lube for added protection.

By the way, you havent mentioned which engine you are referring to

Ameyam