Since they have been recurculating the blowby back into the manifolds there is more residue then in the past and the Exhaust Gas let in by the EGR adds to the mess.

I do not know about dripping oil but there are several "how do I clean my Intake Manifold Threads" for the after 1985 Diesel Mercedes. You might want to ask one of the members who started the Threads what their manifold looked like.

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84 300D, 82 Volvo 244Gl Diesel