🔍 Key Concepts

• Viscosity index (VI) and what it measures in a lubricating oil • How oil viscosity changes with temperature (both when it gets hot and when it gets cold) • Where you would want an oil whose viscosity stays relatively stable over a wide temperature range

💭 Think About

• Ask yourself: Does a high viscosity index oil thin out a lot or only a little as temperature increases? • In what kind of machinery or environment would it be important for the oil to keep nearly the same thickness from cold start-up to normal operating temperature? • Which option describes conditions where temperature is not just high, but changes a lot during use?

✅ Before You Answer

• Be sure you know that high viscosity index = smaller viscosity change with temperature • Eliminate any choices that confuse VI with additives for pour-point or oxidation control • Focus on the choice that clearly involves large swings in temperature, not just ‘high temperature’ alone