GMC 2010 Sierra Owner's Manual page 514

Rear Axle How to Check Lubricant
When to Check Lubricant
It is not necessary to regularly check rear axle 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.
All axle assemblies are filled by volume of fluid during
production. They are not filled to reach a certain level.
When checking the fluid level on any axle, variations in
the readings can be caused by factory fill differences
between the minimum and the maximum fluid volume.
Also, if a vehicle has just been driven before checking
the fluid level, it may appear lower than normal because
fluid has traveled out along the axle tubes and has not
drained back to the sump area. Therefore, a reading
2500HD with 6.0L and 6.2L
taken five minutes after the vehicle has been driven will
appear to have a lower fluid level than a vehicle that
has been stationary for an hour or two. Remember that
the rear axle assembly must be supported to get a true
reading.
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