Final answer:
Engine manufacturers consider the SAE viscosity rating, operating temperatures, and the blend of additives when determining the proper grade of oil for an engine to ensure effective lubrication across the engine’s operating temperature range.
Step-by-step explanation:
When determining the proper grade of oil for a particular engine, manufacturers must consider several factors related to the viscosity of motor oils.
The Society of Automotive Engineers (SAE) has created a rating system from SAE 5 to SAE 50, where the lower the number, the lower the viscosity. Viscosity is important because it affects how well the oil can lubricate the engine parts at different temperatures.
Manufacturers must account for operating temperatures: oils must be viscous enough at high temperatures to maintain lubrication, yet fluid enough at low temperatures to allow for easy starting and effective protection. This has led to the widespread use of multigrade oils, such as SAE 20W/50, which have a versatility to function at both subzero (W for winter) and high temperatures effectively.
Another key consideration includes the blend of additives in the oil, which are designed to modify intermolecular interactions and control the temperature dependence of the oil's viscosity. It's essential not to overuse additives as they may increase viscosity to the point where it could be detrimental, similar to the problems posed by single-grade oils.