I have been using Castrol at least that long as well, and am very comfortable with it. When I didn't use the BMW OE engine oil, I used Castrol Syntec.

The only comment was that their multi-purpose transmission fluid doesn't meet the ZF spec. It is a Dexron fluid that they say is OK to use anyway. Some have had no issues, others have reported issues.

ZF and ZF repair centres know about the problems of putting Dexron fluid in a ZF transmission and specifically warn against it. Yet Castrol imply they have overcome that problem, and also meet just about every other transmission fluid spec at the same time. Amazing accomplishment.

ZF don't make fluid. They just designed the transmission to work with a certain spec of fluid. They control that spec. Lots of companies make fluid to that spec. Castrol doesn't.

I don't think it is a bad fluid. I just think that there are enough variables already with these transmissions and that introducing a non-spec fluid is adding another layer of risk.

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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