The use of VI improvers (VI = viscosity index) enables the production of multigrade engine oils. VI improvers increase or stretch the viscosity of an oil and thus improve the viscosity-temperature behavior. Metaphorically speaking, they are very long, fibrous molecules which, when cold, are present in the oil in a coiled form and offer relatively little resistance to the movement of the oil molecules. As the temperature increases, they unravel, take up a larger volume and form a network of meshes that slow down the movement of the oil molecules and delay the oil from "thinning out" too quickly.

VI improvers / shearing: VI improvers can be sheared under load, i.e. the long molecules are literally torn apart. This is associated with a loss of viscosity. The loss of viscosity is irreversible and this is referred to as permanent shearing. The torn molecules take up a smaller volume and therefore have less of a thickening effect. The shear stability of a lubricant is essentially determined by the quality of the VI improver. High shear loads are present, for example, in the piston ring area (high rotational speeds, sliding speeds, pressures and temperatures).