Viscosity describes the thickness of the oil.

Thicker oil (higher viscosity) is harder to force through a system (pipes, bearings, filters, etc) than thinner oil, so the back pressure through the system increases the pressure of the oil supply from the pump. Difference between pumping molasses vs. water.


Temperature affects viscosity:

Colder oil behaves "thicker" (increased viscosity).

Warmer oil behaves "thinner" (reduced viscosity).