The best diif fluid is probably the
synthetic stuff. I use Mobil 1 gear lube, something like 75w-90. Redline makes a
synthetic gear lube as with some other companies. Even though it is for use in limited slip diff's, I was starting to get some clunking, so I had to add some of the limited diff additive as well.
You fill it up to the level of the fill plug, half way up the diff housing.