🔍 Key Concepts

• viscosity index and what it tells you about how oil thickness changes with temperature • how diesel engine lubricating oil must perform in both cold starts and normal operating temperature • difference between viscosity index and additives like extreme-pressure agents

💭 Think About

• Think about what happens to most oils when the temperature drops very low or rises very high. How does their thickness usually change? • For an engine that will see very cold and very hot ambient conditions, what property do you want in the oil’s viscosity behavior as temperature changes—large change or small change? • Which of the listed choices directly describes how stable viscosity is across a temperature range, rather than adding special chemical properties?

✅ Before You Answer

• Make sure you know that viscosity index is a measure of how much an oil’s viscosity changes with temperature. • Check which option refers to the oil’s behavior over temperature range, not to special additives for gear loads or wear. • Ask: for widely varying temperatures, do we want viscosity to change a lot with temperature, or stay relatively stable? Then match that idea to the correct index choice.