2003 GMC Sierra 1500 Pickup Owner's Manual - Page 433

Four-wheel Drive Lubricant checks in this section also apply to these vehicles. However, there are two additional systems that need lubrication. Transfer Case When to Check Lubricant Refer to the Maintenance Schedule to determine how often to check the lubricant. See Part C: Periodic Maintenance Inspections on page 6- 16. How to Check Lubricant Automatic Transfer Case 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'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 the plug. What to Use Refer to the Maintenance Schedule to determine what kind of lubricant to use. See Part D: Recommended Fluids and Lubricants on page 6- 18. Manual Transfer Case 5-58