Chevrolet 2005 Silverado 1500 Pickup Owner's Manual page 453

Transfer Case
When to Check Lubricant
It is not necessary to regularly check transfer case fluid
unless you suspect there is a leak or you hear an
unusual noise. A fluid loss could indicate a problem.
Have it inspected and repaired.
How to Check Lubricant
Manual Transfer Case
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.
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