Maybe you need it and maybe you don't. When I switched to the Series 2K 75W90, I didn't put any in and I haven't needed it. Others, though, have needed to add some.
There was another thread in which someone said the reason some need the modifier and some don't is that the factory pre-loading (?) on the clutches varies between units. Some are tighter and some are looser. The tighter ones will need some modifier, and the others won't. For that matter, the looser ones could almost use a friction enhancer! That Series 2K seems to be rather slippery stuff!
Go for a drive to warm up the diff, then accelerate through a bunch of left turns. If you hear the left-rear wheel chirping, you need some modifier. I believe someone suggested adding one ounce at a time, then doing figure-8s to work it into the clutches, until the chirping stops.
Fest3er