ORIGINAL: downunder
OTOH here are the Merco instructions regarding castor
.
My guess would be that if the label on medicinal castor says pure castor with no additives then it's probably OK.
Nope I think they mean refined Castor Oil or hydrogenated and esterified Castor Oil (Synthetic Castor Oil). Castrol Mis not a straight pressed castor oil. Castor oil suitable for engines has at least been steam stripped.