I am running 20" OEM X6 style 597 wheels on my wife's 2010 X35d with factory sized tires (Continental DSW's). I had no fitment issues with my truck.

Definitely check the et on those wheels. Not sure which style of wheel you have, but our 597's are:

Front: 20 x 10 ET 40
Rear: 20 x 11 ET 37

I did not have to do any machining of the hubs to get ours to work. I know X6's use a different size front than they do rear hub. I'm not sure which model year(s) this applies to, or if it applies to all of them. So that may be a caveat to the issue you are having. Perhaps the offsets were unsuitable for the ones with larger hubs?

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)