I'm another one who has been cleaning my car engines with my high pressure washer for years. I never did clean the 99E300 this way, let me point that out.
But all my cars with some caution on gas engines, due to the ignition system. I am careful around their electrical systems.
The diesels, I went at it prety hard. Mine is only a 1100PSI unit.

My advise for you is as follows.

Go to the auto parts store and buy the above mentioned GUNK engine cleaner. Drive to a car wash that has the spray type hand held units. Open the hood to the 90 degree position. Spray the engine compartment with the 4-5 cans of gunk. Wait 10-15 minutes and then clean it with the high pressure spray.
You may want to wait for a warm day, you WILL get soaked and dirty. Coveralls are a good idea. Also the car washs are usually closed when its below freezing.

Now you can determine where the oil is leaking from.

Check oil cooling lines, oil cooler, valve cover gasket, engine front systems power steering pump front seal etc. Color of the fluid is important also.

Clean you engine on a regular basis. It easier to work on and solve problems this way.

Again you might want to do this when it above freezing and cover up.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car