I found out something interesting after my last dealer oil change. The display you get at startup is the distance to your next service not necessarily your next oil change.
Right after my oil change the display read 9000 miles which I thought was a bit odd. If you go into the iDrive menu and look for Info Sources and then Car Info (this is on an '08, not sure where it would be on newer iDrives) you can get a listing of the service items such as brake fluid, front brakes, rear brakes and oil change. They will probably list a green check mark next to them. If you scroll through them though it will list the actual mileage to the next service. In my case the front brakes were due in 9000 miles. Oil was at the standard 14000 miles. So basically the startup display will show the lowest number.
That was last fall and it has only gone down to 8000 miles since then but I'm a lot easier on the brakes in the winter.
As for the mysterious oil sensor, I do remember a thread a while ago about this and I don't know if there was ever an explanation for what it actually does. All I know is that I do mid-interval oil changes and the countdown for the oil service doesn't change that I can tell before and after the change. My only guess is that it might speed up the countdown if it senses something funny (like metal) in the oil. But that is only a guess.