GMC 1999 Suburban Owner's Manual page 303

yellowblue
How to Check Lubricant
Automatic Transfer Case
If the level is below the bottom of the filler plug hole,
Manual Transfer Case
you'll need to add some lubricant. Add enough lubricant
to raise the level to the bottom of the filler plug hole.
What to Use
Refer to the Maintenance Schedule to determine what
kind of lubricant to use. See "Recommended Fluids and
Lubricants" in the Index.
6-23