GMC 2005 T-Series Owner's Manual page 165

In order to get an accurate reading, the engine should
be at normal operating temperature, so that the oil
is warm, and the vehicle must be on level ground.
1. If the engine is at normal operating temperature and
the oil is warm, turn off the engine and allow at
least five minutes for the oil to drain back into the
oil pan. If you do not do this, the oil dipstick
might not show the actual level.
If the engine has not been run long enough to bring it
up to normal operating temperature and the oil is
cool, turn off the engine and allow 30 minutes for the
oil to drain back into the oil pan. If you do not do this,
the oil dipstick might not show the actual level.
2. Pull out the dipstick and clean it with a paper towel
or a cloth, then push it back in all the way.
3. Remove the dipstick again, keeping the tip down,
and check the oil level.
When to Add Engine Oil
If the oil is at or below the lower hole at the tip of the
dipstick, then you will need to add at least one quart of oil.
But you must use the right kind. This section explains
what kind of oil to use. For engine oil crankcase capacity,
see Capacities and Specifications on page 5-85.
5-15