What was the agreement when you brought the car in? I've heard of others using the green stuff, but I get the impression that dealer coolant is better for the car. These things should be put in writing when doing business.
Coolant capacity is 10 quarts for the E320. There should be a minimum of 5 quarts coolant added during a changeout. I would've left two gallons.
95 E320 Cabriolet, 131K