I have a 1993 300SD. About 3 yrs. ago my wife became ill and
I was unable to go any farther than the local stores as I had to
spend all my time caring for her. The result was this recent
summer the engine sounded like a dozen men with sledge hammers were inside trying to get out. I then put 1 1/2 qts. of
Marvel Mystery oil in and took it on a run through the hills in 2nd
gear running up to 5000 RPM's. That really helped and with more high RPM running the engine is almost as quite as when new.
The oil must be hot to be effective.