2000 GMC Sierra 1500 Pickup Owner's Manual - Page 330

yellowblue Front Axle When to Check Lubricant Refer to the Maintenance Schedule to determine how often to check the lubricant. See "Scheduled Maintenance Services" in the Index. How to Check Lubricant Automatic Transfer Case If the level is below the bottom of the filler plug hole, you'll need to add some lubricant. Add enough lubricant to raise the level to the bottom of the filler plug hole. Use care not to overtighten plug. What to Use Refer to the Maintenance Schedule to determine what kind of lubricant to use. See "Recommended Fluids and Lubricants" in the Index. If the level is below the bottom of the filler plug hole, you may need to add some lubricant. 6-27