Understanding Viscosity

The viscosity of fluid, also known as dynamic viscosity or absolute viscosity, refers to a fluid’s resistance to flow. This resistance is a result of a shearing stress within a flowing fluid and between the fluid and the container it’s in.

Classifying Viscosity

Many of the unofficial terms that describe fluid viscosity of a flowing fluid (like water) are light, low or thin. Terms that describe a thicker liquid with more flow resistance include heavy, thick, or high (like honey). These descriptors are vague, however, and are hard to measure objectively. Specific description is needed to give us a better understanding of fluid movement.

Fluid movement is impacted by temperature. Consider how honey’s viscosity will increase at lower temperatures and decrease at higher temperatures. Viscosity types are formally classified as kinematic viscosity or absolute viscosity.

Kinematic Viscosity

Kinematic viscosity refers to a fluid’s noticeable tendency to flow. Consider this as how long it takes for a fluid to pour out of a container. The tendency is measured in units that are calculated by volume of flow over time: centistokes (cSt).

Kinematic Viscosity Test (ASTM D445) 

Kinematic viscosity is often defined by high temperatures using the American Society for Testing and Materials (ASTM D445) Viscosity Test. The test utilizes a uniformly calibrated tube, a viscometer, and a heating bath. The bath temperature is either set to 104°F (40°C), standard for industrial lubricants like hydraulic fluids, gear lubricants or compressor oils, or 212°F (100°C), a standard for motor oils. 

Test oil is put into a viscometer and heated to the required temperature of stability. Once the specific temperature has been reached, the oil is drawn into a larger area of the viscometer (recognized by marks on upper and lower) and then able to drain out. Time elapse can be then adapted to centistokes (cSt). In order to be effective and meaningful, cSt must have temperature reported with it.

When comparing viscosities of fluids, you must test them at the same time at the same temperatures or else the comparison is not valid. Though centistokes are the most common kinematic viscosity measurement unit, you can also calculate units known as Saybolt Universal Seconds (SUS or SSU). Centistokes and SUS both measure oil viscosity, but to compare the two would be inaccurate because they’re different units—it would be similar to comparing kilometers and miles. Measuring viscosity in SUS is not common.

Absolute/Dynamic Viscosity

Absolute viscosity (also known as dynamic viscosity) is the resistance of fluid to flowing. This can be thought of as the energy needed to move something through fluid. Consider the example of stirring water with a spoon vs. stirring honey with a spoon—it’s much more energy-consuming to stir the honey. 

Absolute viscosity is typically communicated in units called centipoise (cP). Like cSt and SUS units, a higher cP indicates a greater viscosity.

Brookfield Viscosity Test for Cold Temperatures (ASTM D2983)

The Brookfield Viscosity Test is used to conclude what the internal fluid friction of a drivetrain lubricant at cold temperatures is. For 16 hours, a sample of fluid is cooled in a -40°F (-40°C) liquid bath. After this, the evaluation begins and the energy required to move the object through the oil is determined in centipoise.

Lower cold temperature viscosities (cP numbers) indicate a better performance at lower temperatures.

Cold Crank Simulator Test for “W” Oils (ASTM D5293)

The Cold Crank Simulator (CCS) Viscosity Test determines the internal fluid friction in motor oils with “W” grade specifications. The CCS Test is expressed also in cP units, measuring the amount of energy needed to beat the resistance in lubricants that have been collected at temperatures ranging from 23°F (-5° C) to as low as -31°F (-35°C). The temperature is determined by the SAE “W” classification of whichever oil is being tested. Performance requirements are outlined in the SAE J-300 engine oil viscosity classifications.

The CCS Viscosity test replicates the ability of an engine to turn over at low temperatures. Gauges monitor rotations per minute (rpm), amperage draw and motor input. A universal motor is run at a constant voltage to drive a rotor, which is fitted snugly in a stator and then submerged in the test oil.

Oil viscosity at the given test temperature will determine the speed of the rotor and draw of amperage. Thicker oil will lead to a slower speed and a higher amperage draw. Then the speed and amperage drawn are converted to centipoise.

Much like the Brookfield Viscosity Test, CCS results that have a lower number mean a lower viscosity. Oils that are thicker at lower temperatures (high cP number) usually show higher resistance and therefore require more energy—also showing a higher number on the CSS test. A higher cP number at any temperature relates directly to more energy being necessary to turn the engine over, and it also implies more potential for difficulties starting. The most important aspect of CSS results is likely determining the lubricant’s ability to be circulated at a given temperature and its ability to provide protection from wear.

Viscosity Index

The viscosity index (VI) of a fluid refers to just how much the fluid’s viscosity changes due to temperature. A high VI can mean that the fluid doesn’t change much from temperature alone, and a low VI indicates the opposite.

Fluids with a high VI can provide more protection to important components across a broad range of temperatures by upholding fluid thickness and the appropriate fluid barrier between parts.

Viscosity Index Test (ASTM D2270)

The Viscosity Index Test (ASTM D2270) is determined based on the kinematic viscosity of a given fluid at 104°F (40°C) and 212°F (100°C). The higher the index number, the less of a change between viscosities the fluid has between the two temperatures. A viscosity index number above 95 is regarded as high.

AMSOIL Advantage

Thermal Stability

Synthetic base oils from AMSOIL are more stable thermally than mineral oils. Thermal stability allows the oils to be used for longer, even with increase in temperatures and speed. It can also let oils keep their viscosities steady at low temperatures. A low viscosity oil provides more exceptional cold-weather performance, letting the oil be circulated at cold temperature startups and supplying engine components with the appropriate lubrication to protect them.

High Viscosity Index

AMSOIL lubricants are created specifically to have high viscosity numbers, so the requirement for viscosity index improvers is less necessary. The VI improvers used in AMSOIL lubricants are specific to temperatures, which means they’re only activated when meeting temperature requirements. In the majority of cases, VI improvers help retain thickness at higher temps and have a negligible effect at lower temps. Utilizing viscosity improvers a high shear-stability index allows AMSOIL to achieve ideal cold-weather performance with almost no loss to shear-stability performance. AMSOIL lubricants are designed to resist thinning at high temperatures (high VI) and can suppress the creation of more friction/heat by components that are in contact because of a thinning lubricant.