GMC Sierra 2011 Owner's Manual page 439

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 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.
How to Check Lubricant
2500HD with 6.0 L
I n f o r ma t i o n P r o v i d e d b y :
Vehicle Care
All Other Series and Engines
To get an accurate reading, the
vehicle should be on a level
surface.
For all 4.3 L, 4.8 L and 5.3 L
1500 Series applications, the
proper level is 1.0 mm to
19.0 mm (0.04 in to 0.7 in)
below the bottom of the fill hole,
located on the rear axle. Add
only enough fluid to reach the
proper level.
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