🔍 Key Concepts

• Viscosity is a fluid’s resistance to flow and it changes with temperature • There is a specific rating that shows how well an oil keeps a nearly constant viscosity as temperature changes • Other oil numbers (like API or Saybolt) describe different properties or specific test methods

💭 Think About

• Which of these options specifically refers to how stable the viscosity of an oil remains over a range of temperatures, not just at one test point? • Which options are names of test methods that measure how fast oil flows through a tube, and which is a dimensionless rating of viscosity stability? • Think about what the American Petroleum Institute (API) usually rates for petroleum products—does it normally describe viscosity temperature stability?

✅ Before You Answer

• Identify which choice is used to describe multi‑grade oil behavior (for example, oils that work in both hot and cold conditions) • Separate choices that are time measurements in standardized viscosity tests from those that are index or rating numbers • Confirm which term you’ve seen in relation to how much viscosity changes with temperature, not just how thick a fluid is at one temperature