The type of oil used in a 3.8L Chevy motor depends on the year of the vehicle and the specific recommendations in the owner's manual. There's no single answer. However, generally speaking, a 3.8L Chevy engine (which was a common V6) will use
conventional, synthetic blend, or fully synthetic motor oil meeting the specifications outlined in the owner's manual.
The manual will specify the recommended viscosity (like 5W-30 or 10W-30), and may also list API service classifications (like SM, SN, or SP). Always consult your owner's manual for the correct oil type and viscosity. Using the wrong oil can damage your engine.