If its a box you want to look after, ie a low ratio gearset, or similar, Id go for a synthetic gear oil.
The centre shaft in the transfer box has a really crappy oil feed to the bearings, and can wear quickly if a poor quality oil is used. In the ten or so transfer boxes ive stripped, the wear on the centre bearing pin has been severe.
I use a synthetic gear oil, and a mopar low friction additive, in the vague hope that it helps.
other than that, Id go for a 75 weight oil, try to avoid 90, its like treacle on a cold day.