2008 GMC 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. • 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. 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