A lubricant’s viscosity and how it changes under different temperatures and operating conditions is one of the most important properties...

September 23, 2014

A lubricant’s viscosity and how it changes under different temperatures and operating conditions is one of the most important properties determining performance and protection. Accordingly, Kinematic viscosity is generally the first property listed on an AMSOIL product data bulletin.

Kinematic viscosity, measured using ASTM D445 methodology, determines an SAE oil’s high-temperature viscosity grade (for example, the “30” in 5W-30), while its Cold Crank Simulator (CCS) viscosity, measured using ASTMD 5293 methodology, determines its low-temperature grade (the “5W” in 5W-30).

Why Two Different Test Methods?

Viscosity can be viewed in two ways. Kinematic viscosity is defined by the lubricant’s resistance to flow and shear due to gravity. To illustrate, imagine pouring two containers, one filled with water and the other with honey. The rate at which each fluid flows is governed by its Kinematic viscosity. Since the Kinematic viscosity of water is lower, it flows faster.

Dynamic (or absolute) viscosity, measured by the CCS test, is defined as the lubricant’s resistance to flow as indicated by its measured resistance, best thought of as the amount of energy required to move an object, such as a metal rod, through the fluid. It takes less energy to stir water compared to honey because the dynamic/absolute viscosity of water is lower.

Each test method is designed to replicate a specific operating condition, allowing formulators and end-users to determine the lubricant’s characteristics when in use. The CCS viscosity test evaluates the amount of energy it takes to start an engine at a specified cold temperature; the lower the viscosity grade, the lower the temperature at which the test is performed. The test assigns a value in centipoise (cP), used to determine the viscosity grade. Using Signature Series 5W-30 as an example, its viscosity at -30ºC (-22ºF) can be no greater than 6600 cP to receive a 5W grade. Lower values reflect lighter-viscosity oils.

The Kinematic viscosity test attempts to simulate viscosity at normal operating conditions for a passenger car/light truck. The test is performed at 100ºC (212ºF) and/or 40ºC (104ºF), depending on the grading system being used. The value at 100ºC is used to determine the SAE viscosity grade. The test measures how long the oil takes to completely flow from a viscometer device heated to 100°C. The elapsed time in seconds is converted to centistokes (cSt). Lower values reflect lighter-viscosity oils.

Viscosity Index

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 viscosity change. The Viscosity Index Test (ASTM D2270) is based on the Kinematic viscosity of the fluid at 40°C (104°F) and 100°C (212°F). A fluid whose viscosity does not change much between these two temperatures will have a higher VI than a fluid whose viscosity change is greater. Viscosity index numbers above 95 are considered high. 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.