It is a valve guide sealing problem. Even at that mileage just seals may fix it for a significant period. Significant would be over 30k.

My procedure is to pull all the plugs and look for a specifically heavy carbon buildup. If there is one then do the guide seals on that cylinder first and check the tightness of the guide in the head. If its tight install the seals. It is a 5.5 hour job. If a guide is loose then stop and decide whether to pull the heads now or dump it. A valve job is all labor but it is a bunch of it and the cost will be $2500 -$3000 maybe more depending on a lot of parts issues.

The bottom ends are indestructable. Got a customer with over 900,000 miles on a 91 420SEL (same basic stuff). Two valve jobs the last about 500k ago and numerous valve guide seals but not using oil.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician