Final answer:
In the winter, it is preferable to use motor oil with less viscosity than in the summer, as oils thicken in low temperatures. Viscosity is affected by temperature, so multigrade oils are designed to provide the right viscosity across temperature ranges, functioning as a lighter oil in the cold and a heavier oil in the heat.
Step-by-step explanation:
When selecting motor oils based on their viscosities for different seasons, it is important to consider the temperature changes and their effects on the oil's viscosity. In the winter, when temperatures are low, you would generally use an oil with less viscosity compared to summer. This is because viscosity decreases rapidly with increasing temperatures due to the higher kinetic energy of molecules, which allows them to overcome attractive forces more easily. Therefore, for winter conditions, a motor oil with a lower initial viscosity (e.g., SAE 20W) will still be effective when it thickens due to the cold. In contrast, a more viscous oil suitable for summer heat (such as SAE 50) would become too thick to function properly at lower winter temperatures. Viscosity is crucial in ensuring that the oil can lubricate the engine effectively, regardless of the temperature. It is influenced by intermolecular interactions and molecular size. Multigrade oils, such as SAE 20W/50, have been developed to address this issue. They have additives that modulate viscosity so that the oil functions like a lighter oil (lower viscosity) at cold temperatures and like a heavier oil (higher viscosity) at high temperatures. This versatility makes multigrade oils suitable for use in a variety of climates and driving conditions.