However, generally speaking, for that era of Malibu, 5W-30 was a very common and often recommended viscosity. The owner's manual will specify the *type* of oil (conventional, synthetic blend, or full synthetic) as well. Look for that information in the maintenance section of your manual.
Do not rely on this general information. Always consult your owner's manual for the precise specifications recommended by the manufacturer for your specific 2003 Chevy Malibu engine.