2004 GMC Sierra 1500 Pickup Owner's Manual - Page 462

How to Check Lubricant 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 6-14. Noise Control System Tampering with Noise Control System Prohibited The following information relates to compliance with federal noise emission standards for vehicles with a Gross Vehicle Weight Rating (GVWR) of more than 10,000 lbs (4 536 kg). The Maintenance Schedule provides information on maintaining the noise control system to minimize degradation of the noise emission control system during the life of your vehicle. The noise control system warranty is given in your warranty booklet. These standards apply only to vehicles sold in the United States. To get an accurate reading, the vehicle should be on a level surface. If the level is below the bottom of the filler plug hole, 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. 5-62