Check you allen spanners that came with the truck. One of them should be a very snug fit and not strip. If you still have no joy tap the screw round with a hammer and punch. I have done this a few times with LST screw.

I used WD40 to clean out the grease. As for how much to put in, I followed the instruction on filling savage diffs after spider mod. This said basically fill to just above the cross bars.