I had the same problem,and discovered aluminum flakes in the filter.I have read that some engines,and v8s had bad oil pump sprockets,causing chain to wear into cover.
I di not fool around thou and put a low mileage motor in my car rather than rebuild.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt