2006 Chevrolet Silverado 1500 Pickup Owner's Manual - Page 461

When to Change Lubricant Refer to the Maintenance Schedule to determine how often to change the lubricant. See Scheduled Maintenance (Gasoline Engine) on page 6-4. What to Use Refer to the Maintenance Schedule to determine what kind of lubricant to use. See Recommended Fluids and Lubricants (Gasoline Engine) on page 6-14. 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, located on the transfer case, 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. 5-59