GMC 2006 Sierra - Manual (Page 433)

When to Add Engine Oil
If the oil is below the cross-hatched area at the tip of the
dipstick, you will need to add at least one quart/liter 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 5-133.
Notice: Do not add too much oil. If the engine
has so much oil that the oil level gets above the
cross-hatched area that shows the proper operating
range, the engine could be damaged.
See Engine Compartment
Overview on page 5-14 for
the location of the engine
oil fill cap.
Be sure to add enough oil to put the level somewhere in
the proper operating range. Push the dipstick all the way
back in when you are through.
All Except 8.1L Engine
8.1L Engine
5-21