I use the Ultra-Grey and haven`t had any problems, it keeps the oil from seeping around the tight fit. not any pressure in the Differential.
As we all know there is a vent on the top of the Differential that we all keep clean,.......................right?
As mentioned it is a smooth machine fit, do not jamb in a screwdriver and beat on it with a Hammer to seperate the cover. there is a small hole (as I remember, not under the car at the moment) in the lower corner of the Differential. place a screwdriver/small pry bar in the hole and it will lift the cover off. the cover blocks part of the hole to lift it off with the bar.
Charlie