if you need the synthetic to be legal for a series the castor oil won’t likely test the same and could cause a DQ.
If you’re just running it for practice you will be fine with the castor. It doesn’t burn as clean as synthetic and might cause more carbon buildup than normal.
@Andy_Kutscher is castor oil used for breaking in new engine? Thought I recall reading about how castor differs and I recall them saying also that it was “dirtier”.
Not necessarily for breakin, we used to run castor on our Yamaha’s when it was legal in our club. There is a belief that being “stickier” and “dirtier” that it created better ring sealing and tighter combustion chambers leading to a slight power advantage. The sticky property said to provide better cold / dry start protection too.
If run regularly on long rebuild intervals it requires more frequent cleaning to keep things legal where combustion chambers are measured and to avoid rings from sticking in ring lands.