Chevrolet 2011 Volt Owner's Manual page 379

See
for the location of the
engine oil fill cap.
Add enough oil to put the level
somewhere in the proper operating
range. Push the dipstick all the way
back in when through.
Selecting the right engine oil
depends on both the proper oil
specification and viscosity grade:
Use and ask for engine oils with
the dexos certification mark.
Oils meeting the requirements of
the vehicle should have the dexos
certification mark on the container.
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.