GMC Express 2011 Owner's Manual page 279

This certification mark indicates that
the oil has been approved to the
dexos specification.
This vehicle was filled at the factory
with dexos approved engine oil.
Notice: Use only engine oil
that is approved to the dexos
specification or an equivalent
engine oil of the appropriate
viscosity grade. Engine oils
approved to the dexos
specification will show the
dexos symbol on the container.
Failure to use the recommended
engine oil or equivalent can result
in engine damage not covered by
the vehicle warranty.
If you are unsure whether the
oil is approved to the dexos
specification, ask your service
provider.
Use of Substitute Engine Oils if
dexos is unavailable: In the event
that dexos approved engine oil is
not available at an oil change or for
maintaining proper oil level, you
may use substitute engine oil
displaying the API Starburst symbol
and of SAE 5W-30 viscosity grade.
Use of oils that do not meet the
dexos specification, however, may
result in reduced performance
under certain circumstances.
Viscosity Grade
SAE 5W-30 is the best viscosity
grade for the vehicle. Do not use
other viscosity oils such as
SAE 10W 30, 10W 40, or 20W-50.
Vehicle Care 10-9