For a 1999 Saturn SL2 is 10W-30 motor oil okay to use?

The owner's manual for your 1999 Saturn SL2 is the definitive source for the recommended oil viscosity. However, 10W-30 is a very common and likely acceptable viscosity for a 1999 Saturn SL2. Many vehicles from that era used 10W-30.

But you should absolutely check your owner's manual. The manual will specify the recommended weight(s) and type (conventional, synthetic blend, or synthetic) of motor oil. Using the wrong oil viscosity can negatively impact engine performance and longevity.