I've got that same car, and a few years ago, the oil pressure sending unit started leaking oil. It's screwed into the oil filter base on the lower front side of the engine. Check that out. May only be loosing oil when the engine is running and there's oil pressure.

__________________
past MB rides:
'68 220D
'68 220D(another one)
'67 230
'84 SD
Current rides:
'06 Lexus RX330
'93 Ford F-250
'96 Corvette
'99 Polaris 700 RMK sled
2011 Polaris Assault
'86 Yamaha TT350(good 'ol thumper)