turbo sucks air. any oil inside any part of the air intake tract is residual from the crank case gasses. which is NORMAL and common on 10,000km engines and 500,000km engines.

dripping BLACK oil from the drain holes on the air cleaner could be an indication of something. Since you mentioned that there was a K&N filter in there, I'll bet some one over oiled it and it bled out.

Also if you have a leaking intake manifold gasket, that could put oil in unusual places.

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2002 E430 4matic (212,000km)
2002 ML500 'sport'

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