Rear main seal weep — engine-out fix for a cheap part, improved on 2020+ cars

The rear main seal is the most consistently named mechanical weak point, described as a labor-intensive engine-out repair despite the part itself being inexpensive. Commenters report that facelift/2020+ cars suffer less, and diligent sellers pre-empt buyer concern by replacing the seal and oil separators with updated AMG parts; buyers routinely ask whether it has leaked or been done.