GMC 2005 T-Series Owner's Manual page 166

Notice: Do not add too much oil. If your engine has
so much oil that the oil level gets above the upper
hole that shows the proper operating range,
your engine could be damaged.
5-16
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.
What Kind of Engine Oil to Use
Look for these two things:
•
CI-4 or CH-4
Oils designated as API CI-4 or CH-4 are
recommended for your vehicle. The CI-4 or CH-4
designations may appear either alone, together or in
combination with other API designations, such as
API CI-4/SL.
These letters show American Petroleum
Institute (API) levels of quality.
Notice: If you use oil that does not have this
designation, CH-4, you can cause engine damage
not covered by your warranty.
Notice: Use only engine oils that have one of these
designations CI-4 or CH-4 for the DURAMAX™ diesel
engine. Failure to use the recommended oils can
result in engine damage not covered by your
warranty.