Viscosity is the capacity of a fluid to provide resistance and thus friction to an opposite laminar movement of two adjacent layers. Having a high viscosity means being viscid, whereas a low viscosity is used synonymously to thin fluid.
In short, viscosity can be described as flow resistance.
Viscosity strongly depends on temperature. The indication of the viscosity range for a multigrade oil (e.g. 10W-40) specifies the oil performance at high and low temperatures. The number before the W (10) stands for the viscosity at low temperatures (40°C), whereas the number after the W (40) indicates the viscosity at high temperatures (100°C).
There is a distinction made between the dynamic viscosity (measured in Pa s) and the kinematic viscosity (in mm2/s = cSt), the last one being the quotient of dynamic viscosity and density. Like density, viscosity also depends on temperature and pressure.
The viscosity index of a fluid is determined by its viscosity measured at 40°C and 100°C.
Viscosity was the basis for the first classification system for engine oils in 1911 and was defined in the SAE classification system. Today, viscosity is still one of the most important characteristics of an oil. The development of testing procedures which could better predict the motor behaviour has resulted in viscosity measurements (DIN 51511) at different temperatures and velocity gradient.
There are two measurement categories to specify viscosity:
With Newtonian fluids, shearing stress is proportional to velocity gradient. The proportionality factor is called “dynamic viscosity”. Measure: millipascal second [mPa s], previously centipoise [cP].
Shearing stress = dynamic viscosity • velocity gradient
The Cold-Cranking-Simulator (rotation viscosimeter) is a measuring instrument designed to identify oil viscosity at low temperatures. An electric motor with a constant torque drives a rotor with revolutions that are set according to the viscosimetric characteristics of a particular fluid. Then the dynamic viscosity in mPa s is determined with the help of a calibration curve (made by using standard oils).
At this stage, oils are divided into winter viscosity classes 0W, 5W, 10W, 15W, 20W, 25W. The smaller the number before the W, the thinner the oil at low temperatures.
Viscosity at low temperatures has a direct impact on revolutions of the starter.
The ratio of dynamic viscosity to density at a given temperature. Measure: square millimetre per second [mm²/s], previously centistokes [cSt].
Kinematic viscosity = dynamic viscosity / density
In order to identify the kinematic viscosity, a capillary viscosimeter is used. The design of that viscosimeter can be different, but the measuring principle is identical for all types. Induced by its gravitation, a given amount of oil passes through a defined measuring section of the capillary tube at a particular testing temperature. Determined by the flow time, the kinematic viscosity is measured in square millimetre per second. The division into SAE summer viscosity classes 20, 30, 40, 50, 60 takes place at the testing temperature of 100°C. The higher the number (behind the W), the more viscid the oil at 100°C.
Viscosity Index (VI )
Viscosity of every oil changes according to its temperature. It will be decreasing when the temperature is rising, and increasing when the temperature is declining. Viscosity index is a characteristic for the viscosity/ temperature performance of oils. The higher the number, the less the viscosity of a particular oil will change at temperature variations. The less its viscosity changes conditional upon temperature, the better an oil is suitable for practical use.
Viscosity is the best-known characteristic of lubricating oils. It is the measurement for the inner friction of an oil when flowing. When an oil is cold, its inner friction is high (high viscosity). The warmer the oil, the lower its inner friction will be (low viscosity).
Changes in viscosity conditional upon temperature can vary from oil to oil and are characterised by the viscosity index (VI), a non-dimensional numerical value. Single-range oils have the VI about 100, multigrade oils – up to 150, depending on its range.
SAE Classes in General
Engine and transmission oils for vehicles are characterised by SAE classes, which have also been transferred to the according DIN standards. The following is determined by them: temperatures for viscosity measuring, viscosity threshold values and attribution of classes. Oils that are attributed viscosity threshold values for cold state additionally get the letter “W” (for “winter”) in their classification.
As for hot state, viscosity of engine and transmission oils is determined at 100°C, uniform for all SAE classes. However, this does not come up to real-life conditions of engine oils, thus in many cases, viscosity at high temperatures and under high shearing strain (HTHS ) is additionally appointed to engine oils.
SAE Classes for Engine Oils
Following classes are standardised: 0W, 5W, 10W 15W, 20W, 25W-20, 30, 40, 50, 60
SAE Classes for Transmission Oils
Following classes are standardised: 70W, 75W, 80W, 85W-80, 85, 90, 140, 250
Single-Range vs. Multigrade Oils
If an oil meets the requirements of only one SAE class, then it is a single-range oil, e.g. SAE 10W, 30, 50 for engine oils, or SAE 80W, 90, 140 for transmission oils.
If an oil meets the requirements of both a W-class in a cold state and a class without “W” at 100°C, it is referred to as a multigrade oil, e.g. SAE 0W-30, 10W-40, 15W-40 for engine oils, or SAE 75W-90, 80W-90, 85W-140 for transmission oils.