Final answer:
15W-40 motor oil is designed to function like an SAE 15 at low temperatures and like an SAE 40 at high temperatures, aided by additives that modulate viscosity with temperature.
Step-by-step explanation:
For 15W-40 motor oil, the base oil is not strictly a "15W" or "40" viscosity; rather, it is formulated to behave like an SAE 15 grade oil at low (winter) temperatures and like an SAE 40 grade oil at high temperatures. This multigrade property is achieved by including additives that control the temperature dependence of the viscosity. The oil's ability to maintain adequate lubrication across a range of temperatures is crucial for engine performance, as viscosity decreases rapidly with increasing temperatures due to increased kinetic energy of the molecules, which allows them to overcome intermolecular interactions.