Dunno 'bout castor oil, as that's more of an "official" additive that I let the experts at the mfgs. figure out.

But I DO add about 4-5 drops of Marvel's Mystery Oil to each tank. I have no idea if it's doing anything, but I heard so many testimonials about extended engine life that I am giving it a try.

It's really an automobile product, but people have been using it in LOTS of other applications. It has a very thin consistency, knida like after-run oil, so there's no danger of unwanted buildup or other complications that may arise from adding things like castor oil.

You can get it at most hardware/auto/dept. stores, wherever they sell motor oil.

Dean