ORIGINAL: Sport_Pilot
Actually there is. Plain castor oil will only mix so much with gas. Castrol and others treat it so that larger percentages will mix with gas. I don't know what would happen if you use that with methanol.
I've tried the type of castor that is developed specifically for gasoline engines (Castrol R30 from memory). It would not reliably stay in solution, even when I added up to 5% acetone.
They say that it has "limited" solubility in methanol - but it seems that should be *very* limited solubility.
It's worth noting that to get the benefits of castor, you only need about 3% (by volume) with the balance a good quality synthetic. When I run this small amount of castor (with 9% Cooper's synth) I get *no* carbon build-up even after many hours, the piston crown and cylinder head tend to be discolored to a very light straw color.