Hi, I have the same problem trying to find tyres, except here in Australia we have to keep the same or higher Max Weight Loading of the tyre. fronts 93 and rears 98.
The problem is the 20" tyres have a less weight loading than the oem 19"
X5 F15 M50d White, 20"rims, Yellow Calipers, Ceramic Pads. Better off saying what it does not have and that's a B&O Audio System, otherwise all option boxes are ticked.