From what I've read it oil change count down. I believe it actually based of measured fuel flow into the engine. I think the basic idea is the more fuel the more blow past fumes and the more carbon produced, so the sooner you should change the oil. It does seem to target ~15k in most cars.
Personally I change the oil ~7.5K as well.
An e39 is like a beautiful wife, needs constant attention, lots of money, lets you down at the worse times but you’d never be without it.