GMC 1993 Safari Owner's Manual page 250

Service & Appearance Care
KO704
What Kind of Oil to Use:
Look for three things:
SG
SG must be on the oil container, either by itself or combined with other
quality designations, such as SG/CC, SG/CD, SF, SG, CC, etc. These
letters show American Petroleum Institute (API) levels of quality.
If you use oils that don't have the SG designation, you can cause engine
damage not covered by your warranty.
6-1 2