High mileage vehicles may develop a leak from one or more of the transmission oil seals.Google+
Car Problem Reports
Mercedes-Benz S420 Transmission Oil Seals May Leak on High Mileage Vehicles
Mercedes-Benz S420 Problem
Visitor, 1999 Mercedes-Benz S420, 220,000 mi
Transmission was leaking fluid (green colored). AAMCO in Laguna Niguel, CA replaced the Electrical Connector Body Seals ($13 parts total)and refilled the fluid level. Total: $59 out the door! These guys are great!!