Viscosity is the capacity of a liquid to oppose its deformation with resistance.
- thin oil has a low resistance to deformation;
- thick oil has a high resistance to deformation.
The carrying capacity of a lubricating film is better, however, with a higher viscosity than with a lower viscosity.
The level of viscosity also depends on the temperature. A cold oil is thicker than a warm oil. The extent to which an oil changes its viscosity at changing temperatures varies between oils. It is indicated by the Viscosity Index (VI).
- high viscosity - thick - higher resistance
- low viscosity - thin - low-resistance effect
Through vacuum distillation and the subsequent refining process, there are, in general, three to five base oil fractions of different viscosities produced, so-called “core fractions”. In addition to this, there is a residual fraction that consists of cylinder oil and bright stock, which are extremely viscous products. By mixing together adjacent levels of viscosity, the necessary intermediate levels of viscosity can be produced.