I would just go with the Thule load bars, they mount directly to all of the BMW roof rails I have had and work very well. Benefit is you can use one key for all the locks.

If you do get the BMW load bars, make sure you get the locking ones (other posters have reported that they come both ways, with and without locks).

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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