Since the 96-97 diesels were not turbos, you have a 1998 E300DT. 1998 & later cars have FSS & are the ones with service reset.
If you go past the mileage(not good) then it might take upto 10 times to reset. When you reset the service light the info isn't recorded into the computer, but if the dealer uses the factory tool then the reset is recorded into the computer. If you have any warrenty engine repairs this might be a factor.