Congrats on the X5M!
Good call getting a set of 20's for winter, having separate winter/summer setups is the best option IMO. I'd check the specifics of that tire warranty - does it have to be 1 specific tire brand/model, or just an approved OEM spec tire? I would think if you stick with a Pirelli or Michelin type tire there's no reason BMW would deny a tire claim. I run the Michelin Alpine PA4 winter performance tire... would recommend!
I buy all OEM parts from
www.getbmwparts.com, they're a vendor here.
https://www.getbmwparts.com/oem-part...bC12OC1nYXM%3D
https://www.getbmwparts.com/oem-part...bC12OC1nYXM%3D