Lubrication science
Lubricant properties and the tests behind them
Quick answer
Viscosity, viscosity index, flash point, pour point, acid and base number and more: what each one tells you and the standard test.
Key properties
| Property | What it tells you | Common test |
|---|---|---|
| Kinematic viscosity | How thick the oil is. Decides the film thickness. | ASTM D445 / ISO 3104 (at 40 °C and 100 °C) |
| Viscosity index (VI) | How much viscosity changes with temperature. Higher means more stable. | ASTM D2270 |
| Flash point | Lowest temperature at which vapour can ignite. A safety and volatility clue. | ASTM D92 (open cup) or D93 (closed cup) |
| Pour point | Lowest temperature at which the oil still flows. Matters for cold starts. | ASTM D97 |
| Total base number (TBN) | Reserve alkalinity to neutralise acids. Mainly for engine oils. | ASTM D2896 or D4739 |
| Total acid number (TAN) | Acidic compounds in the oil. Rising TAN means oxidation or additive breakdown. | ASTM D664 |
| Oxidation stability | How well the oil resists thickening and sludge. | ASTM D943 (TOST) or D2272 (RPVOT) |
| Demulsibility | How quickly the oil separates from water. Important in circulating and turbine systems. | ASTM D1401 |
| Foam tendency | How much foam forms and how fast it collapses. | ASTM D892 |
| Density | Used to convert between mass and volume and to spot mix-ups. | ASTM D4052 |
Reading a data sheet
- Use the viscosity at 40 °C to confirm the grade, and the viscosity index to judge behaviour across your temperature range.
- Check the pour point is well below the lowest start-up temperature, not just at it.
- Treat flash point as a safety and volatility check, not a quality ranking.
- Compare TAN and TBN of used oil only with the new oil of the same product.
Properties are not performance
Two oils with the same viscosity can differ widely in additives, oxidation life and water handling. Use the data sheet as a screen, then check the equipment maker's approval list.
Takeaway. Match viscosity first, then check the properties that matter for your duty: pour point for cold starts, oxidation stability for hot service, demulsibility for wet systems.
Put it to work in the tool