1996 GMC Sonoma Owner's Manual - Page 244

How to Check Lubricant What to Use Refer to the Maintenance Schedule to determine what kind of lubricant to use. See "Recommended Fluids and Lubricants" in the Index. Four-Wheel Drive Most lubricant checks in this section also apply to four-wheel-drive vehicles. However, they have 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 "Periodic Maintenance Inspections" in the Index. 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. t