GMC Duramax Diesel 2015 Owner's Manual page 87

10-8 Vehicle Care
American Petroleum Institute (API)
symbol
This doughnut-shaped logo
(symbol) is used on most oil
containers to help you select the
correct oil. It means that the oil has
been certified by the American
Petroleum Institute.
Look for this on the oil container,
and use only those oils that display
this logo.
Caution
Use only engine oils that have the
designation CJ-4 for the diesel
engine. Failure to use the
recommended oil can damage the
DPF and result in engine damage
not covered by the vehicle
warranty.
Viscosity Grade
Use SAE 15W-40 viscosity grade
engine oil.
When it is very cold, below 18 °C
(0 °F), use SAE 5W-40 to improve
cold starting. These numbers on the
oil container show its viscosity,
or thickness.
When selecting an oil of the
appropriate viscosity grade, always
select an oil of the correct
specification. See Specification
earlier in this section.