The type of motor oil for a 1999 Chevy S10 depends on the engine it has. The S10 came with several different engines in 1999. To know for sure, you need to check your owner's manual. It will specify the recommended viscosity (like 5W-30 or 10W-40) and type (conventional, synthetic blend, or full synthetic).
However, a common recommendation for many 1999 S10 engines is 5W-30, but again, consult your owner's manual to be certain. The manual will also specify the correct oil type (conventional, synthetic blend, or full synthetic). Using the wrong type or viscosity can damage your engine.