When determining the oil grade for a vehicle, what should be the primary reference?

The primary reference for determining the oil grade for a vehicle should be the engine manufacturer's recommendations. This is because the manufacturer designs the engine to work optimally with specific oil characteristics, which include viscosity and performance standards. These recommendations typically take into account the engine design, expected operating temperatures, and driving conditions to ensure proper lubrication, reduce wear, and maximize engine efficiency.

Using the manufacturer's guidelines helps to maintain operational reliability and can prevent potential damage to the engine resulting from the use of inappropriate oil grades. While other factors like general guidelines and local weather conditions could provide some insight, they do not replace the specific requirements outlined by the engine manufacturer. Similarly, personal preference of the driver is subjective and may not align with the technical specifications necessary for the vehicle's optimal performance.