GMC 2008 Sierra 1500 Pickup Owner's Manual page 360

How to Check Lubricant
To get an accurate reading, the vehicle should be on
a level surface.
1500 Series
A: Fill Plug
B: Drain Plug
•
When the differential is cold, add enough lubricant
to raise the level from 0 (0 mm) to 1/8 inch (3.2 mm)
below the filler plug hole.
5-48
•
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
Refer to the Maintenance Schedule to determine what
kind of lubricant to use. See Recommended Fluids
and Lubricants on page 6-12.