To be absolutely certain:
* Check your owner's manual: This is the definitive source. The manual will specify the recommended oil viscosity and type (conventional, synthetic blend, or full synthetic).
* Look under the hood: There's often a sticker on the underside of the hood or near the radiator that indicates the recommended oil type and capacity.
If you can't find your owner's manual, a quick online search for "1991 Pontiac Sunbird owner's manual PDF" should yield a downloadable copy. Failing that, specifying the engine size (e.g., 2.0L, 2.5L, etc.) will help narrow down the correct oil type.
In short, don't guess. Always refer to your owner's manual for the manufacturer's recommendation. Using the wrong oil can damage your engine.
