[xDrive50i]

I'm with listerone... You buy an expensive car and want to run it on cheap gas ???

Also, keep in mind that even though the car will run on lower octane gas (e.g. 87), you aren't doing your engine or your pocket any favor. The engine will end up using more low octane gas to do the same work as with high octane gas which means in the long run you don't safe much at all.

Have a look at your manual. The minimum recommended fuel grade is 89 octane (which is usually the medium grade at most gas stations). Lower grade fuels usually have a higher ethanol content (e.g. I think most 87 octane fuels contain 10% Ethanol).