High mileage vehicles may develop a leak from one or more of the transmission oil seals.Google+
Car Problem Reports
Mercedes-Benz S320 Transmission Oil Seals May Leak on High Mileage Vehicles
Mercedes-Benz S320 Problem
Average Mileage: 98,500 mi (77,000 mi - 120,000 mi)
Visitor, 1999 Mercedes-Benz S320, 120,000 mi
Same problem. Ultimately destroyed the transmission. You have to stop *immediately* - don't limp to the repair shop.