The transmission in the 98 is claimed by MB to be a permanent fill. However, there is much debate on this if you search the archives. If the dealer were to follow MB recommendations, there would never be any reason in the lifetime of the car to ever remove the pan.
I had the transmission fluid in my car changed at 70,000 miles to be on the safe side. The only approved fluid is the MB fluid and it is $15 a quart, never use anything else besides this in the tranny.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record