VISCOSITY is a measure of a fluid's resistance to flow, to which many of the qualities of oil are directly related.
It is a very good indicator as to whether or not the oil’s life has been depleted; hence one of the key characters in deciding when to drain oil. Film strength is directly related to viscosity and is a major value in hydrodynamic lubrication.
For monitoring purposes, viscosity is normally measured at 40 degC (100 degF) with the Australian accepted unit of the Centistoke (cSt), however some imported oils use different units, such as the US Centipoise (cP) or Saybolt seconds (SUS) which is the preferred US unit. Sometimes we also measure viscosity at 100 degC (212 degF) which in many cases is closer to the operating temperature of the oil and can be a more useful value in higher operating temperature machinery such as engines.
The difference in viscosity between 40 degC and 100 degC is also used to calculate the 'Viscosity Index' (VI) of the oil which is a single unit measure of the change in viscosity relative to temperature change. The higher the VI number, the less the viscosity variation between the two temperatures. A high VI oil will have a typical value of around 140 and low values can approach 0.
Viscosity 'shear' with use as the oil condition deteriorates leading to a drop in viscosity. In some engines, this condition will be evident as an increase in oil consumption which means the oil condition is deteriorating with use. Shearing is less common with today's high quality oil technology than it has been in the past, however using multi-grade engine oils (15w40) in some high performance hydraulic systems will lead to immediate shearing.
Our main interest with viscosity in used oil analysis is the movement in viscosity of the sampled oil from new. As a general rule, a movement of plus or minus 10% up to 20% of original viscosity (depending on operation standards set) is a good indicator when oil change is due.
If viscosity increases the cause will be either particle contamination or oxidation (burning).
If viscosity drops, the cause will be either fluid dilution or shearing. Always check viscosity when monitoring oil condition.
Never ignore viscosity movements, there is always a reason. Identify and act on the cause to preserve your oil's integrity which will keep your wear levels reduced.