1998 GMC Envoy Owner's Manual - Page 267

Four-wheel Drive Transfer Case When to Check Lubricant Refer to the Maintenance Schedule todetermine how often to check the lubricant. See "Periodic Maintenance Inspections" in the Index. How to Check Lubricant 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. Refer to the Maintenance Schedule to determine what kind of lubricant to use. See "Recommended Fluids and Lubricants" in the Index. 6-20