Don't worry. I just climbed the Mt. Washington, NH with my 99 E430 Sport on July 1, 2001. Initialy, I had shifted to third gear. However, I later found out the car was at the second gear most of the time. I found out by shifting to second to see if the RPM would drop.
Also I have found out the car does a great job going down hill. The gear does hold the speed as you decending, the transmission will even downshift temporary to reduce speed.
99 E430 Sport