There is actually a special type of grease that goes into the box. Lighter than axle grease but heavier than 90wt oil. I don't remember what it is but I think it's been discussed on this forum many times so you could probably do a search and find lots of information about it. I do know it's definitely not 90wt oil.
I used a synthetic grease, CV-2, though I later discovered that For made a specific grease for the application. I thought about cleaning out the box and repacking it the Ford box grease but have not experienced any problems with the lube I used. The CV-2 goes for $10 a tub while a good Hi-temp moly should run 2 to 4 a tub. I never checked with the Ford dealer regarding the price of their box grease.