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Oil Analysis And Testing

Oil Viscosity

The Analysis of Engine Oil Through Viscosity Testing

Engine Oil Viscosity is a measurement of the resistance of a fluid being deformed by either shear stress or extensional stress. It can also be thought of as a measure of fluid friction. There are many different types of ways of expressing a fluid’s viscosity. Here at OCLS, we choose to calculate the fluid’s kinematic viscosity using a series of Houillon Viscometers. The kinematic viscosity is the ratio of the viscous force to the inertial force (or density).

It can be defined as:

  • ν = Kinematic Viscosity (cSt [Centistokes])
  • µ = Absolute Viscosity (cP [Centipoise])
  • ρ = Density (g cm³ [grams per centimetre-cubed])

The analysis of engine oil through viscosity testing

Simply speaking, the lower the viscosity value, the thinner the oil will be. Which follows that a high viscosity value indicates a thicker oil. For example, water has a viscosity of about 1 cSt. An average engine oil has a viscosity of about 100 cSt.

Viscosity is a very important property of oil that allows it to protect the internal workings of a machine by creating a thin film of oil between moving parts. Viscosity can be affected by many factors and so regular monitoring is essential. The test is generally carried out on industrial machinery such as engines, hydraulics, gearboxes etc.

Oil is a non-Newtonian fluid, which means that its viscosity is not constant – it cannot be described by a single number – it varies. For example, a temperature increase by only 5°C can cause the viscosity of some fluids to double! OCLS checks viscosity routinely at 40°C, but other temperatures up to 100°C are used. Oil viscosity can be affected by many factors and so regular monitoring is essential.

Typically OCLS determines kinematic viscosity at 2 temperatures (40°C and 100°C using ASTM D445.) This allows a viscosity index to be calculated, where viscosity at other temperatures can be determined.

Using the Wrong Viscosity Oil:


Oil viscosity too high

  • Inadequate flow to components
  • Energy consumption losses occur
  • Excessive heat generation causing varnishes and sludge’s
  • Poor oil flow in cold-start conditions


Oil viscosity too low

  • Lack of protective oil film between components
  • Excessive wear
  • Oil film fails at high temperatures or high loads
  • Increase in friction, resulting in heat generation
  • Internal or external oil leaks