If you had a oil leak for a while as a result a lot of the under body is cover with this "dirty oil stuff". But you fix the leak. Will old oil eventually dry up and fall off? If so how long months, years? Can someone speak from experience. Had engine oil and power steering oil leak on my wife 2008 ML350. I think I fixed the leaks (finger cross).

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Silver Honda Accord, 2006
Silver G500, 2003
Silver SLK-320, 2002
Black ML-320, 2000
Bule Porsche 993 Targa , 1997
Silver Merkur XR4Ti, 1987