In Germany, the X5 M comes on 20'' rims too. Unfortunately it is very risky to swap tires (summer to winter and vice versa) on the same rim. The reason is simple: you can damage the tire (especially low profile, high speed index tires are at risk) and this damage is usually invisible to the eye. It is recommended to keep two tire sets, one for winter and one for summer. I know this means a lot of money but if you don't want to go for the expensive BMW rims, there are alternatives out there on the market which are cheaper (rims).