How does engine oil viscosity relate to temperature?

• A. Viscosity remains constant across temperatures.
• B. Viscosity indicates oil thickness; multi-viscosity oils (e.g., 5W-30) flow well at cold temperatures and provide protection at operating temperatures.
• C. Viscosity only matters for diesel engines.
• D. Viscosity is unrelated to temperature.

Answer: (B)

Engine oil viscosity describes how thick or thin the oil is, and temperature directly affects that thickness. When oil is cold, it thickens and doesn’t flow quickly, which can left the oil film from reaching engine parts fast enough on startup. As the engine warms up, the oil becomes thinner, which helps it circulate, but if it get too thin it may not maintain a strong protective film under operating loads. Multiviscosity oils, like 5W-30, are formulated with additives that help the oil flow well at cold temperatures (the winter rating) while still providing enough viscosity at normal operating temperatures to protect moving parts. The numbers reflect this behavior: the first part (before the W) indicates cold-flow performance, and the second part indicates viscosity at operating temperature. So, oil viscosity is temperature-dependent, and multi-viscosity oils are designed to perform well across a range of temperatures.