Chevrolet 2006 Silverado 1500 Pickup Owner's Manual page 461

Automatic Transfer Case
To get an accurate reading, the vehicle should be on a
level surface.
If the level is below the bottom of the filler plug hole,
located on the transfer case, you'll need to add
some lubricant. Add enough lubricant to raise the level
to the bottom of the filler plug hole. Use care not to
overtighten the plug.
When to Change Lubricant
Refer to the Maintenance Schedule to determine how
often to change the lubricant. See Scheduled
Maintenance (Gasoline Engine) on page 6-4.
What to Use
Refer to the Maintenance Schedule to determine what
kind of lubricant to use. See Recommended Fluids
and Lubricants (Gasoline Engine) on page 6-14.
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