Try finding a mild citric acid cleaner.

Heck, just go down to the carwash with $5. Use the white wall cleaner on the engine and spray the underside well. You can remove the lower shroud under the engine easily and leave it at home.
Then blast it with the soapy cleaning high pressure spray until you get rid of the oil.

Go home and jackup the car- support the car properly with jackstands. Then look while it is running for oil leaks.

Generally on a M103 engine:
front timing upper cover
valve cover gasket
Head gasket
Rear crankshaft seal
Transmission front pump seal
Front crankshaft seal

Your probably looking at 1-2 of these problems. Some like the timing cover are regular service items-maybe every 60,000 miles or so.
I added the transmission b/c this does happen with age. If the fluid is redish in color coming from the bellhousing between the engine and transmission- that would be an indication of a front seal
Michael

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Michael McGuire
83 300d
01 vw A4 TDI
66 Chevy Corsa
68 GMC V6 w/oD
86 300E