The Viscosity Index (VI) of an oil is a numerical value that indicates how much an oil’s viscosity changes with temperature. It helps describe how stable or unstable an oil is across a range of temperatures.
🧪 What is Viscosity Index?
- Low VI: Viscosity changes a lot with temperature
- High VI: Viscosity stays relatively stable as temperature changes
Example:
- An oil with low VI might be thick when cold and too thin when hot.
- An oil with high VI flows better in cold and maintains thickness when hot — ideal for modern engines or equipment under varying conditions.
📊 Typical Viscosity Index Ranges
| VI Value | Classification | Base Oil Type Example |
|---|---|---|
| < 35 | Very Low | Naphthenic base oils |
| 35–80 | Low | Group I (solvent-refined mineral oils) |
| 80–120 | Medium / Standard | Group II, basic Group III |
| > 120 | High | Group III+, PAO, synthetic esters |
| > 150–200+ | Very High / Super High | PAO, advanced synthetic blends |
🧰 Why Viscosity Index Matters
| Application | Importance of VI |
|---|---|
| Engine oils | High VI ensures better protection across hot/cold temperatures |
| Hydraulic oils | High VI maintains system efficiency and pressure stability |
| Gear oils | Prevents shear thinning at high load/heat |
| Cold climates | High VI helps start-up lubrication and reduces wear |
🔍 Real-Life Comparison
| Property | Low VI Oil | High VI Oil |
|---|---|---|
| Cold-start behavior | Thick and sluggish | Flows easily |
| High-temp stability | Thins too much | Maintains protective film |
| Equipment wear | Higher risk | Lower risk |
| Shear stability | Lower | Higher |
🧪 Viscosity Index Improvers (Additives)
- Often added to multi-grade oils (e.g., SAE 10W-40)
- Common chemistries:
- Olefin copolymers (OCP)
- Hydrogenated styrene-diene polymers
- Polymethacrylates (PMA)
Used to increase VI and enable broader temperature ranges without sacrificing oil performance.
📝
| Question | Answer |
|---|---|
| What is VI? | A measure of how much viscosity changes with temperature |
| High VI means? | Oil stays more stable from cold to hot temperatures |
| Who needs high VI oil? | Engines, hydraulics, or gearboxes in variable temp conditions |
| Can VI be improved? | Yes, using viscosity index improver additives |
Would you like a chart comparing VI and temperature behavior, or help choosing the right VI for your lubricant product line?
Engine Oil Viscosity Index: Understanding the Differences and Effects of Different Viscosity
Engine oil, as the “blood” of engine operation, its performance and quality are crucial to the normal operation of the engine. Among the performance indicators of engine oil, viscosity is a very important parameter. The difference in engine oil viscosity index not only affects the performance of the engine oil, but also affects the operation of the engine.
First, let’s understand what engine oil viscosity is. Viscosity refers to how easily the oil flows, which is how thick the oil is. The higher the viscosity of the engine oil, the worse the fluidity, and vice versa. The viscosity of engine oil is generally expressed by the viscosity grade established by SAE (Society of Automotive Engineers), such as 5W-30, 10W-40, etc.
The engine oil viscosity index refers to the degree to which the viscosity of engine oil changes at different temperatures. The higher the viscosity index, the smaller the viscosity of the engine oil changes with temperature, that is, the better the viscosity stability of the engine oil. On the contrary, the lower the viscosity index, the greater the viscosity of the engine oil changes with temperature, and the worse the viscosity stability.
So, what are the effects of differences in engine oil viscosity index?
First, for starting and running the engine. Engine oil with a lower viscosity index has poor fluidity at low temperatures, which may cause difficulty in starting the engine and even cause engine wear. Engine oil with a higher viscosity index can still maintain good fluidity at low temperatures, which helps the engine start smoothly. At high temperatures, engine oil with a higher viscosity index can maintain a more stable viscosity, reduce engine wear, and improve engine operating efficiency.
Secondly, regarding the engine’s fuel consumption and emissions. Oil with a lower viscosity index may produce greater internal friction when the engine is running, resulting in increased fuel consumption. Engine oil with a higher viscosity index can reduce internal friction and reduce fuel consumption due to its better viscosity stability. At the same time, engine oil with a higher viscosity index can also help reduce engine emissions and reduce environmental pollution.
In summary, differences in engine oil viscosity index have an important impact on engine operation and performance. When selecting engine oil, we should choose an engine oil with an appropriate viscosity grade and viscosity index based on factors such as engine type, operating environment and temperature to ensure normal operation of the engine and extend its service life. At the same time, we should also pay attention to the quality and brand reputation of the engine oil, and choose engine oil with reliable quality and stable performance to provide better protection for the engine.