Try to stay away from these tranny additives--it might be a quick fix, but in the long run your tranny will suffer even more. Try changing the fluid a couple of times, or bite the bullet and get it fixed.
Sorry to hear about what you've heard, but i've had my car to this guy three times knowing myself what the problem was (thanks to MBshop) and he was on spot with the diagnosis all three times. My friend had also taken his former 1985 380SE with a moonroof problem--everyone he took it to told him he would need a new motor--big bucks!! Steve had the car, and the next day the moonroof was operating perfectly at a cost of $75. From my experience, this guy is on point with his ****, and he only works on MB's--not BMW's, Audi's, Lexus, etc...
I know of another good mechanic in the brooklyn area that I've heard good things about, but I've never had my car taken there for service. Write me an e-mail and I should have the name and address of this shop--supposed to be a an experienced MB foreman in there, a Turkish gentleman.
I'm sure you'll find a superior shop sooner or later, with an excellent MB mechanic that will take care of your needs, and your 300E.
Maybe Yal can jump in here and name some good independent MB tech in the NY area.
Monotoned Smoke Silver
97 Style Replica E420 Rims 205/55/16