The early trucks were equipped with convention petroleum lube in the front axle. But, apparently some 4x4 trucks had problems with their front axle when driven in cold environments. This caused a change in lube recommendations for later trucks. And, GM produced a service bulletin for the older trucks, which allows them to use synthetic lube (once a specific part is swapped for one that is compatible with synthetic).

Bulletin No.: 03-04-19-001B, Date: September, 2004

Use Synthetic Axle Lubricant, P/N 12378261 or P/N 89021677 (Canadian P/N 10953455 or P/N 89021678). Each of these part numbers is the same axle lubricant, with differences in packaging only. Fill to a level between 6 and 9 mm (0.25 in to 0.375 in) below the bottom of the fill plug.

If the vent hose connector is black in color, it will need to be changed so that the 0-ring seal is compatible with the synthetic lubricant. Install vent connector, P/N 12479390. Follow the Front Drive Axle Vent Hose Connector Replacement procedure in the Service Manual.