No other figure states more about an oil’s properties and its quality than the viscosity index (VI). The VI is a dimensionless number, i.e. it has no physical unit. It allows for a better comparison of the viscosity behavior of different oils based on temperature. The viscosity index is important in order to ensure, e.g., the best possible lubrication for machinery, as temperature changes occur during operation.
Figure 1: Viscosity index – schematic
The viscosity of lube oil does not give any relevant information unless the temperature, at which it was measured, is known. Temperature is the most influential parameter for the viscosity change of oil. For any oil used under changing temperature conditions, it is important to know the change of viscosity in relation to the change of temperature. For this purpose, the viscosity index12 for base stocks and lube oils was developed by Dean and Davis from Standard Oil in the year 1929. At this time, no multi-grade oils and no synthetic oils were available. For the VI scale, two limit points were set. Oils with low temperature-dependent viscosity change (HVI-oils, refined from Pennsylvania crudes, paraffinic oil) were on the high end of the scale. Their VI was indicated with 100, which represented the best VI. Oils with significant viscosity change (LVI-oils, refined from Texas gulf crudes, naphthenic oil) represented the low end. Their VI was indicated with 0 – this was the worst possible VI. The VI values related to mineral oils. Lube oils were then compared to these benchmarks. If the oil was similar to the paraffinic oil, a VI of 100 was assigned; if it was similar to the naphthenic oil, a VI of 0 was assigned. In the middle, a VI of around 50 would be assigned. To increase the VI to values higher than 100, new base oil types and special additives were developed later on.
In the early VI systems, the temperatures for viscosity measurement were 100 °F and 210 °F, corresponding to 37.78 °C and 98.89 °C. The Fahrenheit temperature scale is still frequently used in the Anglo-American area. Today, temperatures of 40 °C and 100 °C are common. Initially, the viscosity was measured in Saybolt Universal Seconds (SUS). Today, the kinematic viscosity in mm²/s is used for the VI calculation. To compare SUS (ASTM D88) with mm²/s, an online calculator, such as the Anton Paar calculator for ASTM D88 / Saybolt universal viscosity can be used.
Compared to water, which has nearly the same flow behavior over a wide temperature range, oil changes its viscosity significantly with changing temperature. Additional influences such as oxidation, contamination, and pressure during operation have an impact on the viscosity. Furthermore, the viscosity change over temperature is not linear. It follows a double logarithmic function.
A low VI expresses a considerable change of viscosity with change of temperature. Such oils are highly viscous at low temperatures and rather thin at high temperatures. A high VI means the opposite: a small change of viscosity over a wide temperature range. When choosing oil for a special purpose, e.g. to use it for lubrication within an internal combustion engine, the temperature-related viscosity change must be considered as it differs between oil types. Oils which have the same kinematic viscosity at 40 °C can have very different properties at 100 °C. To obtain the required temperature-related viscosity properties of oil, VI improvers, also known as viscosity modifiers, are added to the base oil. The maximum achievable VI depends on the used base oil type as well as on the type and concentration of VI improvers in the oil. The VI of common oil types ranges from -60 to over 400. The content of viscosity modifiers is approximately 5 % to 20 %. To get detailed information on the viscosity properties of engine oil at various temperatures for different SAE grades, see the following Anton Paar table: Viscosity of engine oil.
For an overview of the viscosity indexes of different fluids see the following table:
| Oil / fluid types | Viscosity index |
|---|---|
| Mineral oil | 95 – 105 |
| Multi-grade oil | 140 – 200 |
| PAO oil | 135 – 160 |
| Ester | 140 – 190 |
| Vegetable oil | 195 – 210 |
| Glycol | 200 – 220 |
| Silicone oil | 205 – 400 |
Table 1: Viscosity index of different fluids3
Figure 2: Influence of viscosity index improvers: mono-grade vs. multi-grade engine oil
Formulated lube oils contain various additives. One of the most important groups comprises viscosity index improvers4 (= VII)/viscosity modifiers. These are mainly oil soluble polymers or copolymers. They can be used for both mineral and synthetic base oil types.
VI improvers work – expressed in a simplified way – via shape change. The polymer molecules are small and coil-shaped, or folded when cold. In that state, they do not increase the oil’s viscosity, as there is rather low friction on the wetted surfaces in the engine and in the liquid itself. With rising temperature, the molecules expand and unfold or uncoil. Consequently, they increase the friction in the liquid and compensate the decrease of viscosity that is caused by the higher temperatures. The impact of a VII on the overall system lube oil further depends on the molecular weight of the viscosity index improver5.
VI improving additives also have some drawbacks. They are sensitive to ageing caused by repeated mechanical shearing, which disrupts the molecule chains. Over time, additives lose their ability to act as thickener in the oil at high temperatures. Using polymers with higher molecular weight would improve the thickening properties, but they show less resistance to mechanical shearing. Polymers with lower molecular weight are more shear-resistant, but they do not increase the viscosity at high temperatures effectively enough. This is why they must be added in larger quantities. Without viscosity index improvers, it would not be possible to formulate today’s modern multi-grade lube oils. Figure 2 displays how VI improvers influence the oil’s temperature-dependent viscosity change.
The practical example in this figure shows two mono-grade oils for use in engines of road vehicles. SAE 10 has a lower viscosity at low temperatures than SAE 40. Roughly said, the first oil is for use in cold surroundings: it is the “winter” oil. SAE 40 is for use in warm surroundings: it is the “summer” oil. By adding VI improvers (and other additives) to SAE 10, it is possible to formulate a multi-grade oil that contains both properties: the SAE 10W-40. This oil has properties of both oils: the good pumpability at low temperatures of SAE 10, and a thicker, more stable, oil film at increased temperature of SAE 40. By using a multi-grade oil, there is no more need to change the engine oil with the season. For details regarding SAE (SAE International; former Society of Automotive Engineers) viscosity classification see SAE J300 and SAE J306 standards or our article on SAE viscosity grades for a specification of oils over a broad temperature range.
Widely used materials are e.g.: olefin copolymers (OCP), polyalkyl methacrylates (PAMA), poly isobutylenes (PIB), styrene block polymers, methylmethacrylate (MMA), polybutadiene rubber (PBR), cis-polyisoprene (a synthetic rubber), polyvinyl palmitate, polyvinyl caprylate, copolymers of vinyl palmitate with vinyl acetate, and various others.
VI modifiers are mainly used in multi-grade engine oils, gear oils and automatic transmission fluids, power steering fluids, hydraulic fluids, and also in greases. Most of these use cases are related to means of mobility, such as road vehicles, watercrafts, and aircrafts, as there are significant changes in operating temperature.
For machinery operated at more constant conditions, oils with a lower VI are sufficient.
Figure 3: Formulated lubricant
Lube oils for automotive internal combustion engines must have a high viscosity index. The lube oil must be liquid enough at low temperatures to allow a smooth cold-start of an engine and must be easily pumpable for a fast oil supply to all lubricating points. Further, its viscosity must be high enough at high temperatures to maintain a load-bearing lubrication film. Nowadays, engine oils provide a VI in the range of approximately 140 to 200. Find more information in the report: Viscosity Index of Base and Lube Oils with SVM 4001.
Figure 4: Power generation – transformer oils ensure excellent heat transfer
These oils should have a low viscosity index to improve the natural convection at elevated operating temperatures. Low viscosity at higher operating temperatures leads to turbulent flow, which increases the heat transfer. A low viscosity index ensures efficient cooling performance of a transformer oil. These oils provide a VI in the range of approximately 100 to 60, and even lower.
Figure 5: Quenching – an important part of the hardening procedure in metal working
Quench oils have a rather low viscosity index, often below 100. They must have a low viscosity at high temperatures to ensure smooth and evenly distributed wetting of the metal surface to be hardened. Quench oils must remove the heat rapidly but evenly from the immersed work piece to allow a perfect hardening process.
They typically have a VI around 90.
There are two standard procedures to calculate the viscosity index: ASTM D22706 and ISO 29097. These are the current methods for calculating the VI for petroleum products used in civil and military applications.
The viscosity index is important both for new oils and for in-service oils. The latter is used to check the degradation of the viscosity modifiers, which usually causes a decrease of the VI.
The viscosity index is defined only for petroleum products with a kinematic viscosity higher than 2 mm²/s at 100 °C.
The measurement should be performed according to ASTM D70428 or ASTM D4459 respectively ISO 310410 or IP 7111. In cases where it is not possible to measure the oil at the standard temperatures, the kinematic viscosity can be measured at different temperatures. These temperatures should be as close as possible to the original temperatures, but as widely spread as possible. From these measured values, the kinematic viscosity at 40 °C and 100 °C can be determined according to ASTM D34112.
For viscometers which allow the measurement of temperature scans: Due to the behavior of the polymer molecules, it often makes a difference whether the viscosity measurement is performed with rising temperatures or with descending temperatures. The viscosity readings of a measurement from 40 °C to 100 °C can differ from those of a descending measurement from 100 °C to 40 °C. In such a case, it is recommended to perform the measurement and the repeat measurement always in one direction, preferably from lower to higher temperature.
To calculate the VI, the basic parameters L and H, which are related to high and low viscosity index oils, both at the same temperature of 40 °C, must be determined. Using these parameters and the measured viscosity of the oils at 40 °C and 100 °C, the viscosity index can be calculated. The calculation follows different formulae depending on the balance of the basic parameter H and the measured kinematic viscosity at 40 °C.
For further details on VI calculation, refer to ASTM D2270 respectively ISO 2909.
To get a fast VI calculation from two known viscosities, you can use the following calculators in the Anton Paar Wiki: VI from kinematic viscosity at 40 °C and 100 °C or VI from kinematic viscosity at two free temperatures.
Viscosity index is the most important parameter indicating an oil’s temperature-related flow properties. Selecting an oil without considering its VI for a certain application can e.g. lead to premature wear and costly damage of machinery. To learn more about viscosity determination, it is recommended to