1998 GMC Sonoma Owner's Manual - Page 263

What Kind of Engine Oil to Use Oils recommended for your vehicle can be identified by looking for the "Starburst" symbol, Thissymbol indicates that the oil has been certified by the American Petroleum Institute (API). Do not use any oil which does not carry this Starburst symbol. If you change your own oil, be sure you use oil that has the Starburst symbol on the front of the oil container. ~. RECOMMENDED SAE VlSCOSlTV GRADE ENGINE OILS SELECT THEiOWEST SA€ VISCOSITY GRADE OIL FOR THE EXPECTED TEMPERATURE RANGE... HOT WEATHER LOOK FOR MIS SYMBOL If you have your oil changed for you, be sure the ail put into your engine is American Petroleum Institute certified for gasoline engines. You should also use the proper viscosity oil for your vehicle, as shown in the following chart: DO NOT USE SAE 2ow-50 OR ANY OTHER GRADE OIL NOT RECOMMENDED 6-13