FAA Powerplant Written Practice Test

What characteristic of a lubricating oil is the viscosity index based upon?

1. Its volatility

2. Its rate of change in viscosity with temperature change

3. Its chemical composition

4. Its color

The correct answer is: Its rate of change in viscosity with temperature change

The viscosity index is a measure that indicates how much a lubricating oil's viscosity changes with temperature. In essence, it reflects the oil's ability to maintain consistent viscosity across a wide temperature range. A higher viscosity index means that the oil will exhibit less change in viscosity with changes in temperature, making it more stable and effective for varying operating conditions. This stability helps ensure that the oil provides optimal lubrication, protecting engine components from wear and tear, particularly under extreme heat or cold. Volatility, chemical composition, and color do not specifically address how viscosity changes with temperature, which is the fundamental aspect defined by the viscosity index. Thus, understanding the relationship between viscosity, temperature, and the viscosity index is crucial in selecting the appropriate lubricating oil for a given application.