Viscosity is a measure of fluids resistance to flow. Often defined as an ISO or SAE viscosity grade (such as VG 220 or SAE 15W/40). Typically measured at 40 or 100ºC.
Each application will have a viscosity range suited to the task. Too high a viscosity can lead to lubricant starvation, wear in circulating pumps, increased temperatures and reduced efficiency. If viscosity is too low, the components will not be sufficiently separated resulting in excess friction and causing wear on the machinery.
A change in viscosity could be due to:
- Contamination with water/fuel/solvents/very small particles
- Oxidation or ageing of the oil
- Incorrect oil
- Mechanical shearing of the oil