My oil is good and clean.
Just playing devils advocate and maybe I'll learn something ...
What would make it dirty?Motor
oil is designed to suspend combustion contaminants and shows it by changing color. It's for sure a sign things are working right and not at all a sign the oil is "worn out."
But what contaminants are there in a rear drive?
Methinks if there are contaminants there's a seal problem rather than an oil problem.
Yes, there is "breathing" and it's just SOP to check the oil should there have been an encounter with lots of water that could have entered via breather. Wouldn't wait for "2nd tire" to make that check. Otherwise, don't see much reason to vary from BMW change specifications.