You don't need to use BMW coolant, as long as it meets BMW specs. To make it easy you could just use one of the many coolants on the BMW approved list. There are others that work, but BMW lists the ones they have tested, for those that don't want to check the specs.

I just recently used OE coolant, not because it is necessarily the best, but because different coolants can react with each other, and I knew what was in there last. Lowest 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