1998 GMC Suburban Owner's Manual - Page 317

Front Axle When to Check and Change Lubricant Refer to the Maintenance Scheduleto determine how often to check the lubricant and when to change it. See "Scheduled Maintenance Services" in the Index. How to Check Lubricant Automatic Transfer Case I f 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 t o the bottom of the filler plug hole. 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.