It seems very strange to me that O'Donnell fuel would reject castor oil, unless O'Donnell fuel isn't made from the normal ingredients of methanol and nitromethane. Castor oil is used in methanol fuel because it is uniquely good at dissolving in alcohols; most oils don't dissolve in alcohols very well. So for O'Donnell fuel to reject castor oil, there must be something very strange about its formulation. Maybe it's already saturated with some other type of oil and it can't hold any more?