I've found SHOUT! to work really well. 'Bout 3 bucks at Walmart -- it doesn't foam up and it dissolves the oil really well, without being too harsh on your coolant system.
Also, be sure to replace all of the "soft" oil soaked hoses that balloon on the ends. They will eventually fail on you if not replaced -- especially check the heater hoses.
'91 300E, 211K miles