Hi.
Just an opinion, but..
I know very succesfull sportsmen who use castor oil in their 4-strokes (mostly Saito .56). For example Beringers and Luc Dessaucy. I pretty sure they use at least 50% of castor in the oil mix.
And what I have observed, these engines run the best compared to the other 4-strokes seen in competitions over here.
But what I've learned from Luc especially, you must prepare yourself to dismantle & clean the engine more often, and these guys also rework their engines in order to minimize the friction between the moving parts (mostly re-lapping and polishing of functional surfaces).
I've seen many people go the other way with all synthetic with varying results, but these engines seem to be missing something in the harmony.
Lauri