Chevrolet 2011 Express 1500 Cargo 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.
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.
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.