I found oil in the air filter housing in my 240 about a year ago & assumed that it was a severe blow-by problem. Luckily, the problem turned out to be the vacuum pump. It was blowing oil through the hose that runs from the pump to the top of the housing. I'm not sure if your 1981 has this hose . My 1982 SD doesn't ... it has the single hose pump. I paid $8.00 for the pump rebuild kit. But, I found a single hose pump from a 300d at my local salvage yard for $5.00 (price included the compatible air filter housing).

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1980 Mercedes 240D
1982 Mercedes 300SD
1985 Mercedes 300D
1993 Dodge Cummins Turbo Diesel