GMC 2003 C4C042 Owner's Manual page 282

When to Add Engine Oil
If the oil is at or below the ADD line, then you'll need
to add at least one quart of oil. But you must use the
right kind. This part explains what kind of oil to use.
For engine oil crankcase capacity, see "Capacities and
Specifications" in the Index.
NOTICE:
Don't add too much oil. If your engine has so
much oil that the oil level gets above the proper
operating range, your engine could be damaged.
Be sure to fill it enough to put the level somewhere in
the proper operating range. Push the dipstick all the way
back in when you're through.
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What Kind of Engine Oil to Use
Look for these two things:
D
-
CH 4
Oils designated as API CH
-
vehicle. The CH 4 designation may appear either
alone or in combination with other API designations,
-
such as API CH 4/SJ or CH
These letters show American Petroleum Institute
(API) levels of quality.
NOTICE:
If you use oil that doesn't have this designation,
-
CH 4, you can cause engine damage which is not
covered by your warranty.
-
4 are best for your
- -
4/CG 4/SJ.