2007 GMC Envoy Owner's Manual - Page 430

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 1/2 inch (12 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. Bulb Replacement For the proper type of replacement bulbs, see Replacement Bulbs on page 435. For any bulb changing procedure not listed in this section, contact your dealer. Halogen Bulbs What to Use To determine what kind of lubricant to use, see Recommended Fluids and Lubricants on page 517. {CAUTION: Halogen bulbs have pressurized gas inside and can burst if you drop or scratch the bulb. You or others could be injured. Be sure to read and follow the instructions on the bulb package. 430