A gentleman on the BMW message board posted the follwing question and response from Castrol: What group is the base stock on your fully synthetic oil, III or IV? Is any part of the additive package derived from petroleum? Thank you. REPLY: In 1998, Castrol upgraded SYNTEC, by switching to a new and specially engineered hydroisomerized base stock. SYNTEC is not made with petroleum base stock. Just as PAO is derived from complex chemical reactions starting with ethylene gas, SYNTEC's base stock is synthesized using sophisticated processes and chemical reactions. The molecular structures of the starting compounds are fundamentally altered to produce a fluid with vastly improved properties capable of attaining the highest levels of formulated synthetic lubricant performance. Castrol Consumer Relations. Any comments??? How can they say that "Syntec is not made from Petroleum Base stocks"?