I highly recommend "Auto-RX" oil treatment. My 300CE was gunked up so bad that my indie was stunned. We had to dig the oil filter out. It looked like it had never been changed! Auto*RX recommends 2 treatments for this type of engine. You can tell how it is working as the filter is changed twice for one oil change. It amazed us to see the flakes of gunk in the filter. It is NOT a solvent cleaner. My engine interior is now like new. I also noticed an increase in power although that might be wishful thinking.
Anziani
'93 300DE190K
'95 E420 120K