Yeah, definatly! If you change it on a regular basis your probably ok and in fact I was just thinking of doing it with my next oil change but if in water dump the cover. If the dealership charged you to take off the cover then I would not be happy as they saved themselves some work and charged you for it.

I used diff fluid that had the friction mod in it, I think it was a Castrol product. Adding it when not needed I doubt will hurt anything but your wallet.

 

Glad that they made right on their mistake!