For what it is worth, this happens wih my 1998 E320 as well. It only happens in the winter when it is cold enough such that things condense in the oil fill cavity. My car has never been on a trip shorter than 40 miles in the 100K miles on the car.

The only difference in warmer seasons is that the sludge swirls around in the crankcase until it ends up in the oil or gets ventilated. I clean it out every week in the winter and forget about it the rest of the time, other than to change the oil reasonably frequently to get the crud out of the crankcase.

I believe that it is piston blowby accumulating in the crankcase, that either condenses in the oil fill upper tube, blends in with the oil, or is ventilated. Either way, it is not desirable, but not terminal, just a chronic condition of the model.