janmarie, Castrol Syntec isn't what it used to be, look for my article here. Seems they changed their formula from pure synthetic to group III petroleum base stocks. Was sued by Mobil 1 because of it. They have legal rights to claim their syntec is full synthetic, but it's a play on words and meanings. I recommend Amsoil, or Red Line & Mobil 1, other than Castrol Syntec. I see others writing about European Syntec being a better formula, don't know, but I live here in USA!
Hydrocracked Basestocks: Affecting an Industry
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BMW 325i