JEB,
Jim Smith is right about this one. I have no Idea if it only applies to the original owner however.
The cause is from the use of dino oil and changing it at the 10,000 to 12,000 mile or more intervals that the FSS system was giving. The oil rings stuck to the pistons and allowed them to excessively consume oil. I personally rebuilt at least 50 of the M112 engines for this problem. Most of them were in the 163 chassis (ML) but some were in others like the 210 you are experiencing this with.
This was not necessarily the fault of the owner. The dealers were installing dino oil in them until this problem came up and Mom Benz said "whoops". Then they started using Mobil 1. We tried multiple engine flushes and nothing worked except to replace the piston rings. Sometimes we had to install blocks fitted with pistons due to gouges in the cylinder walls.
Good Luck
Brian

__________________
Mercedes Benz Master Tech
ASE Master Technician
Diagnostic Technician
Owner of "Behind the Star Inc."
Raleigh NC.
www.behindthestar.com
brian@behindthestar.com