GMC 2007 Sierra 1500 Pickup Owner's Manual page 530

How to Check Lubricant
To get an accurate reading, the vehicle should be
on a level surface.
530
If the level is below the bottom of the filler plug
hole, located on the front axle, you may need
to add some lubricant:
•
When the differential is cold, add enough
lubricant to raise the level to 0 to 3/8 inch
(9.5 mm) below the filler plug hole.
•
When the differential is at operating
temperature (warm), add enough lubricant to
raise the level to the bottom of the filler
plug hole.
What to Use
To determine what kind of lubricant to use see
Recommended Fluids and Lubricants on page 629.