I use plain old straight 50wt. Motor oil, fill it up to the bottom of the filler hole with the rig sitting level. That is about 2.5>3 qts. Usually, depends upon how gunked the interior is and how long ya let it drain, and if hot or cold.
The equivalent gear oil would be a straight 80 or 90 wt. Or a 85/90. There are no brass/bronze/zinc/aluminum components as "wear/load" elements in these transfer cases, so typical modern gear oil additives have no effect in the long term.
The viscosity index/ratings for motor oil and gear oils are not the same scale. For example, a 40wt. Motor oil is approximately the same viscosity as an 80wt. Gear lube.