During May 2000 I bought a 1995 E420. When I did the oil and filter change for the first time, I put in exactly 8 1/2 quarts as the manual calls for. The oil level came up to midway between minimum and maximum. That is the same level it always was at whenever I checked it prior to the oil change.
Before that oil change I was concerned that the car had burned some oil since it wasn't at the max level. Based on this experience with the E420, I now think Mercedes intends for the middle point to be the proper oil level.