Fluid viscosity, sometimes referred to as dynamic viscosity or absolute viscosity, is a fluid’s resistance to flow, which is caused by a shearing stress within a flowing fluid and between a flowing fluid and its container.
Some of the informal terms used to describe the viscosity of a relatively free-flowing fluid, such as water, include thin, light and low. Terms such as thick, heavy or high suggest a fluid with strong 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 fluids move.
Temperature affects how a fluid moves. Imagine how the viscosity of honey would greatly increase at temperatures near freezing and decrease near boiling temperatures. To understand these different reactions, viscosity types are scientifically classified as kinematic viscosity or absolute viscosity.
Kinematic viscosity describes a fluid’s visible tendency to flow. Think of this as the time it takes to watch a fluid pour out of a container. This tendency to flow is expressed in units suggesting the volume of flow over time, called centistokes (cSt).
Kinematic Viscosity Test (ASTM 0445)
Kinematic viscosity is commonly determined under high temperatures using the American Society for Testing and Materials (ASTM 0445) Viscosity Test. 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 104°F (40°C), which is typical for industrial lubricants such as hydraulic fluids, compressor oils or gear lubricants, or 212°F (100°C), which is typical for motor oils.
The test oil is placed in a viscometer and heated by the bath to the specified stable temperature.
When the specified temperature is reached, the oil is drawn into a wider area within the viscometer, which is identified by upper and lower marks, and allowed to drain out. The elapsed time can be directly converted into centistokes (cSt). To be relevant, the cSt number must be reported along with the temperature at which it was determined.
When comparing fluid viscosities, the fluids being compared must be tested at the same time and at constant temperatures, or the comparison is invalid.
Although centistokes are the most common unit of measurement when determining kinematic viscosity, results also may be reported in units known as Saybolt Universal Seconds (SUS or SSU).
Even though identical test temperatures may have been used to determine the oil’s viscosity in both centistokes and Saybolt Universal Seconds, the two should never be compared at face value because they are different units of measure. To do so would be similar to comparing distances in miles and kilometers. Viscosity reported in SUS is becoming increasingly rare.
Absolute viscosity, or dynamic viscosity, is a fluid’s resistance to flow. 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 known as centipoise (c}’). As with cSt and SUS units, the higher the number of cP units assigned to a fluid, the greater its viscosity.
Brookfield Viscosity Test for Cold Temperatures (ASTM 02983)
The Brookfield Viscosity Test is used to determine the internal fluid friction of a drivetrain lubricant at cold temperatures. A fluid sample is cooled in a liquid bath at -40°F (-40°C) for 16 hours. The sample is then evaluated, and the force required to move an object through the oil is recorded and converted to centipoise.
Lower cold-temperature viscosities (lower cP numbers) reported with this test indicate improved performance at cold temperatures.
Cold Crank Simulator Test for “w” Oils (ASTM D5293)
The Cold Crank Simulator (CCS) Viscosity Test is used to determine the internal fluid friction in motor oils with a “w” grade designation. This test is calculated in cP units as well, and measures the amount of energy required to overcome the resistance present in a lubricant that has been collected at temperatures from 23°F (-5° C) to as low as -31°F (-35°C), depending upon the anticipated SAE “w” classification of the oil being tested. Performance requirements to meet SAE “w” grades are outlined in the SAE J-300 engine oil viscosity classifications.
The CCS Viscosity Test simulates an engine’s ability to turn over at cold 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 closely fitted inside a stator and immersed in the test oil.
The viscosity of the oil at the given test temperature determines the speed of the rotor and amperage draw; thicker oil results in slower speed and more amperage drawn. Speed and amperage drawn are then converted to centipoise.
Like the Brookfield Viscosity Test, CCS results showing a lower cP number indicate lower viscosity. Oils that are thicker at low temperatures (high cP number) tend to exhibit more resistance and require more energy to pump and circulate and display a higher cP number on the CSS test. A higher cP number at a given temperature is directly correlated to a greater amount of energy required to turn an engine over, and it also indicates a greater potential for starting difficulties. Most importantly, CCS results suggest a lubricant’s ability to be circulated at a given temperature and its ability to provide wear protection.
The viscosity index (VI) of a lubricating fluid refers to how much the viscosity of the fluid changes due to temperature. A high VI indicates the fluid undergoes little viscosity change due to temperature fluctuations, while a low VI indicates a relatively large amount of viscosity change.
Fluids with a high VI provide more protection to critical components over a wide range of temperatures by maintaining fluid thickness and the necessary fluid barrier between parts.
Viscosity Index Test (ASTM D2270)
The Viscosity Index Test (ASTM 02270) is based on the kinematic viscosity of the fluid at 104°F (40°C) and 212°F (100°C). Fluids whose viscosities do not change much between these two temperatures will have higher viscosity indices than those whose viscosity changes are greater. Viscosity index numbers above 95 are considered high.
AMSOIL Advantage Thermal Stability
AMSOIL synthetic base oils have better thermal stability than mineral oils. Thermal stability permits the oils
to be used longer, even as speeds and temperatures increase. It also allows oils to retain their viscosities at low temperatures. Lower-viscosity oil provides better cold-weather operation, allowing the oil to be quickly circulated at cold-temperature startups and supply engine components with the proper lubrication to keep them protected.
High Viscosity Index
AMSOIL lubricants are formulated to have naturally high viscosity indices, so the need for viscosity index improvers is reduced. The VI improvers used in AMSOIL lubricants are temperature specific, meaning they are activated only when certain temperature requirements are met. In most cases, VI improvers help maintain thickness at higher temperatures while having minimal effect at low temperatures. By using viscosity improvers with a high shear-stability index, AMSOIL is able to achieve optimal cold-weather performance with virtually no loss to shear-stability performance.
AMSOIL lubricants resist thinning at high temperatures (high VI) and can suppress the generation of additional friction and heat generated by components in contact due to a thinning lubricant.
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SLS Note: For those of you who think that 0w16 or even 0w20 oils are too thin, in January 2015 SAE, at the request of auto manufacturers, approved SAE 12 and SAE 8 viscosity grades. At the time of the request, Honda had already completed successful tests using oils with similar parameters.
AMSOIL offers 0w16 oils for those vehicles that require it and when the market is there, I’m sure they will offer a 0w12 or even a 0w8 viscosity grade that will protect better than the competition.