The Significance of Viscosity Index in Motor Oil and Lubricants

Understanding an Oil’s Viscosity Index and Its Impact on Engine Operation

Viscosity is a measure of a fluid’s resistance to flow. For engine oils, viscosity determines how easily the oil can flow through the engine and lubricate moving parts. Low-viscosity oils flow more easily and quickly, while high-viscosity oils are thicker and flow more slowly. However, viscosity alone doesn’t fully describe how an oil will perform in an engine. For that, you need to know its viscosity index (VI).

What is VI?

The change in an oil’s viscosity due to a temperature change is called the viscosity index (VI). It is a critical property of lubricating oils, as it helps to understand how an oil will perform across different temperature ranges.

The viscosity index is calculated based on the viscosity measurements of the oil at 40°C and 100°C. The higher the VI, the less the oil’s viscosity changes with temperature. Special additives, known as viscosity index improvers, are often used in oil formulations to enhance the VI.

• High VI— Oils with a high viscosity index experience less change in viscosity with temperature fluctuations. This means they remain relatively stable across a wide temperature range, providing consistent lubrication and protection.
• Low VI— Oils with a low viscosity index exhibit more significant changes in viscosity with temperature variations. They may become too thin at high temperatures or too thick at low temperatures, leading to potential issues in engine operation.

Understanding the Impact of Viscosity Index on Engine Operation

Impact of VI on Cold Starts

High VI Oils—  These oils remain relatively fluid even at low temperatures, ensuring quick and efficient lubrication during cold starts. This reduces engine wear and makes it easier for the engine to start.
Low VI Oils— These oils can become too thick in cold conditions, making it harder for the oil to circulate quickly. This can lead to increased wear during start-up and potentially longer crank times.

Operating Temperature Range

High VI Oils— Maintain optimal viscosity over a broad temperature range, providing consistent protection and performance. This is especially important for engines operating in varying climates or under different load conditions.
Low VI Oils— May thin out excessively at high temperatures, leading to inadequate lubrication and increased wear. Conversely, they may become too viscous at low temperatures, leading to poor circulation and increased resistance.

Fuel Efficiency

High VI Oils— By maintaining a stable viscosity, high VI oils reduce internal friction within the engine. This can improve fuel efficiency and reduce energy loss, contributing to better overall engine performance.
Low VI Oils— Variable viscosity can lead to inconsistent lubrication and increased friction, potentially reducing fuel efficiency.

Engine Wear and Tear

High VI Oils— Provide consistent lubrication and protection, reducing wear on engine components and potentially extending engine life.
Low VI Oils— Fluctuating viscosity can lead to periods of either insufficient or excessive lubrication, increasing the risk of wear and tear on engine parts.

Oil Change Intervals

High VI Oils— Typically allow for longer oil change intervals due to their stability and resistance to viscosity breakdown.
Low VI Oils— May require more frequent changes as they are more susceptible to viscosity changes and breakdown over time.

Why is a higher viscosity index better?

The best oils are the ones with the highest VI because they remain stable with the least amount of viscosity variation over a wide temperature range. So they provide more consistent flow and lubrication to the moving parts.

A lubricating fluid or oil with a higher VI means the oil is more stable across a broad temperature range than the same “weight” oil with a lower VI specification.

For more information on viscosity index, see this article.

©, 2019 Rick Muscoplat