I had the same problem on the 4.5 Mike. Found a replacement oil cap gasket at PartsShop or FastLane for something like $0.30. Easy replacement (but kinda tough getting the cross piece back in). Tightened up the cap just enough and now no more oil running down from the top of the valve cover. Might see if they list a replacement gasket for the SD.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.