A chief benefit of using castor oil is that it will protect the engine against a lean mix and the very high temperatures generated much better than any other oil, including synthetics. Nothing has quite the same protective qualities that castor oil provides.
It will be far safer to run the engine slightly rich, but NEVER lean when using alternative lubricants. This means we need to remember that the engine "unloads" a bit in flight and the mix will go a bit leaner as it will when the tank level drops low.
What exactly does a lean run have to do with the oil and why? I'd like to see the view point as this is published all over the place but never really explained. And yes, I know what happens to castor when heated, but does that really apply?