I just got an ASP 1.20 4-stroke engine and discovered that the manual specifies castor oil fuel only for run in, and castor/synthetic mix after that.

There is nowhere around here that actually sells castor oil fuel, and while I can mix my own fuel (basically a necessity round here because the pre-mixed fuel is very poor quality) I was just wondering how necessary the castor actually is, and if there's a specific type of castor needed?