I've had great luck with Auto-RX. My '93 300CE had a crudded up oil system. The filter was black and SOLID with sludge when I first got the car. I followed their instructions and it cleaned it up beautifully. Even stopped a couple of leaks. I now add 3 oz per oil change.

http://www.auto-rx.com/index.html

Anziani
'93 300CE 180K
'95 E420 117K