As jt20 said a compression test would give you more info. It would tell you if it is a worn out Cylinder/Piston/Rings issue or not.

It also could tell you something I think was not mentioned here is a leaking Head Gasket. It could show up as 1 obviously low cylinder compared to the other cylinders or 2 low cylinders next to each other with the others being obviously higher.

FSM = Factory Service Manual (If bought from the Mercedes Classic Center $19.99 + $7 shipping; you get it on 2 CDs).

As for what to do- if your Engine is worn out the choice is to Rebuild the engine, Replace the Engine with a used one or drive the Car until it will not run any more (use the Oil Fill Cap hole and make something to add some more ventilation so that the pressure is not high enough to force Oil out of the Rear Main Oil Sea).

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84 300D, 82 Volvo 244Gl Diesel