My transmission was overfilled is what I was told by the dealership and I was told that could cause severe damage to my transmission! It is now slipping.

 

How far overfilled?

 

It certainly can. Expecially in older transmissions, but even newer ones aren't immune. Im not a transmission god, so I wont go into details, but I know that it can cause erratic shifting and blown seals. I guess that depends on how its designed. Some have holes in them so they can be overfilled and some are "sealed for life".

 

a litre overfilled!

 

Level is to be checked with engine running after full warm up of the transmission. If it is overfull at that point, remove some until the correct level is indicated on the dip stick while running.
Good luck.