The OEM oil filters used in old Mercedes diesels do NOT filter soot at all. If you keep changing your oil every 3 Kmiles (highway), your engine is pretty clean and I believe you can change the oil filter every other time.

The major reason why you should change oil frequently on diesels is because you want to remove the soot being suspended in the old oil, due to the fact that oil filters are not designed to trap soot. Soot particles are too small for them, but they are harmful enough for engine innards if the soot concentration in the oil exceeds 2% (That's the maximum allowable soot limit for MB diesels).

Eric