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

It will never be clean again, but you will be able to tell fresh leaking shiny wet oil from old dust/dirt caked dried up oil. Clean up around the source of the leak with a solvent and a rag so that you can see if it is continuing to leak.

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Old Blue - '83 240D 6 speed manual trans, OM648
Green Machine - 2001 Europa G500
'87 300SDL Bought on Peachparts
'02 E320 4matic wagon

Patchouli Wagon - 2004 E500 - SOLD
333k mile 97 BMW 750il V12. - SOLD
The Californian - 85 300TD - SOLD
Daily Super Sedan - 03 Audi S8 - SOLD
Family Truckster - 2012 VW Touareg TDI - SOLD

The oil will never burn away or fall off, it will attract dirt/mud/grime and just become a greasy coating forever. As mentioned above, fresh leaks will be shiny and "wet". If it bothers you, buy some degreaser and go visit the local DIY carwash and go to town on it. You can get it very clean again with some elbow grease and a good degreaser.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

go to the spray and was and spray it off with the pressure washer

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