If I had to choose between an engine designer/manufacturer and their recommendation, or the recommendation of a company selling oil, I'd go with the engine manufacturer. Especially when the engine company holds the risks of both the lease residual value, and the warranty/extended warranty fund. I have used Castrol Syntec, but I usually use the BMW 5w-30 made by Castrol, which isn't Syntec. I note that Mobil I now recommends following the manufacturer's change recommendations, up to 15,000 miles, unlike Castrol which is still pushing the 4000 - 6000 mile intervals that the motor oil industry has been promoting for years (although to be fair, they used to promote 3000 mile intervals).

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue