All castor oil that is virgin first pressing is the same oil. Medicinal castor is refined a little more (filtered tighter). It's all the same oil. Very little second/multiple pressing oil makes it into liquid oil production. A lot of that is used in cosmetics and stuff. I've used castor from fuel companies and apothecary companies alike. Same weight, color, smell, and performance in engines. Medicinal castor is more expensive, but if all you're doing is spiking your fuel, it's fine. If using medicinal castor, ensure it has no additives or sugar in it. Racing castor - like Klotz Benol - is a polymerized castor and as such isn't virgin castor. It will not mix with some fuels - such as O'Donnell's speed blend. Stick with virgin castor and you'll be fine.

I buy two gallons at a time from bulkapothecary.com for $17/gal. it works as well as the castor from SIG and S&W fuels.