1999 GMC Sierra 1500 Pickup Owner's Manual - Page 318

Senrice d& Appearance Cam Front Axle When to Check and Change Lubricant: Refer to the Maintenance Sched,ule to determine howoftentocheckthe MaintenanceServices" in lubricantandwhen to changeit.See"Scheduled theIndex. How to Check Lubricant: PO231 If the. level is belowthebottomofthe fillerplughole,you'llneedtoadd somelubricant. If the differential is at operating temperature (warm),add f enough lubricant to raise the level to the bottom o the filler plug hole. If thedifferentialiscold, add, enoughlubricanttoraisethelevel (12 mm) below the filler plug hole. to 1/2 inch What to Use: Refer to the Maintenance Schedule to determine what .kind of lubricant to use.See"RecommendedFluidsandLubricants'' in theIndex. 1 ' 6-40