BTW, there is only a gasket between the aluminum upper oil pan and the iron sump. There isn't a preformed gasket between the block and aluminum oil pan. There's a specific sealant that MB shops use though a number of sealants should do an adequate job. There's no way around separating the pan from the block and thoroughly cleaning both surfaces, though.

As MBeige suggests, thoroughly clean the engine rather than take it apart. Have it pressure washed or even steam cleaned then watch it daily/hourly/in real time as necessary to find the source. If you take it apart now, you'll have a lot of dirty parts and possibly insufficient forensic ability to find the source.

Sixto
87 300D