I believe the capacitance of the oil is measured. I also thought that the M111 engines included this feature. If not, it wouldn't recognize an interim oil change or topping up with fresh oil.
Isn't there also some time limit such as 13 months between changes or am I incorrect on this assumption.
I always disliked this kind of feature on my BMW's. At least with Mercedes you can reset it w/o taking it to a dealer.