BMW dealerships use Castrol full synthetic - from Europe. They DO NOT use Castrol Syntec. Use of Castrol Syntec instantly voids your warranty. Only a limited number of oil brands meet the BMW requirements.
I understand that the Mobil 1 sold at Walmart is full synthetic and does meet the BMW standards. Look it up in the owners manual to be sure. It is much less expensive than the Castrol sold at the stealerships, but about double the cost of Castrol Syntec.