It's called Mercedes-Benz alkaline cooling system cleaner (MB part no. 001 986 21 71). According to the E-Class Owner's Bible, a 3% solution works well. Run the engine for 10 minutes before completely flushing the cleaner out of the system with clear water. The heater must be on and running to clear the oil from the heater core.

The process should be repeated until all the oil is removed.

BTW, ask your dealer what THEY use in this situation.