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.