When pressure is applied to a liquid, the viscosity typically increases. The only exception is water below 32 °C and some water-based solutions. It is assumed that the effect is caused by the free volume of the liquid, which is reduced when pressure is applied. This leads to an increase of inner friction and therefore to an increase in viscosity.
The increase of viscosity depends strongly on the material and molecule structure. Compared to temperature, however, pressure influences are significantly smaller. Many liquids with a simple molecule structure show minor viscosity changes in a moderate pressure range up to several hundred bar, and therefore, the influence of pressure can often be neglected for technical applications. Materials with longer molecules can be affected more significantly by increased pressure. Especially for branched molecules, which have additional free inner volume, the pressure effect can be increased. The pressure influence is often non-linear and increases further with increasing pressure. This is especially relevant for lubrication, where very high pressures can appear. Also, in the field of oil recovery, pressure influences can become significant.