GMC 1996 Suburban Owner's Manual page 278

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How to Check Lubricant If the level is below the bottom of the filler plug hole,
you'll need to add some lubricant.
If
the differential is at operating temperature (warm),
add enough lubricant to raise the level to the bottom of
the filler plug hole.
If the differential is cold, add enough lubricant to raise
the level to 1/2 inch (12 mm) below 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.
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