CHF 11s if your reservoir is so labelled. ATF otherwise. No mixing, they are not compatible.

If ATF, I would use any Dexron from any reputable major brand. Or Mercon. Not a synthetic, as what is already in there isn't a synthetic. There is no reason to hunt for a special ATF, in this case it is being used only as a hydraulic fluid and the ingredients added to different ATF fluids for different specs are essentially irrelevant in a power steering system.

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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