The 'Viscosity' of a oil is a way of measuing the the resistance to flow of a engine oil caused by friction within the molecules that make up the oil. Some of the informal terms used to describe the viscosity of a relatively free-flowing liquid such as water include thin, light, and low. Terms such as thick, heavy, or high suggest a liquid with a high resistance to flow, such as honey. However, these terms are general and difficult to measure. More specific classifications give us a better idea of how liquids move.
Temperature affects how a liquid moves. Imagine how the viscosity of honey would increase sharply at temperatures near freezing and decrease near boiling temperatures. To understand these different reactions, types of viscosity are scientifically classified as kinematic viscosity or absolute viscosity.
Kinematic viscosity describes the visual representation of a liquid flowing. Think of this as the time it takes to watch a liquid flow out of a container. This flow property is expressed in units of flow volume over time called centistokes (cSt).
Kinematic viscosity is typically determined at high temperatures using the American Society for Testing and Materials viscosity test (ASTM D445). This test uses a uniformly marked or calibrated tube called a viscometer and a heating bath. The temperature of the bath is set at either 40°C (104°F), which is typical for industrial lubricants such as hydraulic fluids, compressor oils or gear lubricants, or at 100°C (212°F), which is typical for motor oils. The test oil is placed in a viscometer and heated to the specified stable temperature by the bath.
When the specified temperature is reached, the oil is drawn into a larger area within the viscometer, identified by upper and lower marks, and allowed to drain. Elapsed time can be converted directly to centistokes (cSt). The cSt number is given together with the temperature at which it was determined.
When comparing liquid viscosities, the liquids being compared must be tested at the same time and at constant temperatures, otherwise the comparison is invalid.
Although centistokes(cSt) are the most common unit of measurement when determining kinematic viscosity, results can also be reported in units known as Saybolt Universal Seconds (SUS or SSU).
Although identical test temperatures were used to determine the viscosity of the oil in both centistokes and Saybolt Universal Seconds, the two should never be compared at face value as they are different units of measurement. This would be similar to comparing distances in miles and kilometers. Viscosity given in SUS is becoming rarer and rarer.
Absolute viscosity or dynamic viscosity is the resistance to flow of a liquid. Think of this as the energy required to move an object through a fluid. It takes little energy to stir water with a spoon; however, stirring honey with the same spoon requires significantly more energy. Absolute or dynamic viscosity is generally expressed in units called centipoise (cP). As with cSt and SUS units, the higher the number of cP units associated with a liquid, the greater its viscosity.
The viscosity index (VI) of a lubricating fluid refers to how much the fluid's viscosity changes with temperature. A high Viscosity Index indicates that the fluid experiences a small change in viscosity due to temperature changes, while a low Viscosity Index indicates a relatively large change in viscosity.
High viscosity index lubricants provide more protection for critical components over a wide temperature range by maintaining the lubricant thickness and necessary barrier between parts.