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Air Compressor Oil Viscosity - Good to Know!

At SA Performance, lubricants are our specialty. Our distributors rely on us to provide quality oil products, but also to provide the expertise needed for oil sampling programs and analysis.

According to the Society of Tribologists and Lubrication Engineers (STLE), "Viscosity is one of the oil's most important physical properties. It is often the first parameters measured by most oil analysis labs because of the importance to oil condition and lubrication."


An oil's viscosity is defined as its resistance to flow and shear due to gravity.

Viscosity Index

Viscosity Index (VI) is an arbitrary measure for the change of viscosity with variations in temperature. The lower the VI, the greater the change of viscosity of the oil with temperature. The higher the VI, the lower the change of viscosity with oil temperature. It is used to characterize viscosity changes with relation to temperature in lubricating oil. Lubricants with a higher VI provide better protection at higher temperatures.

Viscosity Modifiers

Viscosity modifiers are polymeric molecules that are sensitive to temperature. At low temperatures, the molecule chain contracts and does not impact the fluid viscosity. At high temperatures, the chain relaxes and an increase in viscosity occurs.

Viscosity Index Improver Disadvantages

Unfortunately, viscosity index improvers do have some drawbacks. The primary disadvantage is that they are susceptible to mechanical shearing. The additive is repeatedly sheared which causes a loss of its ability to act as a more viscous fluid at higher temperatures. Higher molecular weight polymers make better thickeners, but tend to have less resistance to mechanical shear. Lower molecular weight polymers are more shear-resistant, but do not improve viscosity as effectively at higher temperatures.

So, why is this important to your customers (and to you as a industrial air compressor lubricant distributor)?

It is important to know the basestock attributes of your aftermarket supplier lubricants. The American Petroleum Institute (API) categorizes basestocks by their sulfur content, level of saturates and viscosity index (VI). There are five API basestock classifications:

  • Group I Solvent Dewaxed Basestock is the least refined – usually a mix of different hydrocarbon chains. These oils are generally used in application without high performance demand but have significant advantages when it comes to additives. These are designated with SN followed by a viscosity number. The SN stands for Solvent Neutral.

  • Group II Hydroprocessing and Refining Basestock is common in commercial mineral-based motor oils. They perform acceptably in terms of volatility, oxidative stability and flash/fire points but not as well in terms of cold-start viscosity, extreme pressure durability and pour point. Greater purity means that the basestock and the additives in the finished product will last much longer.

  • Group III Hydroprocessing and refining Basestock is the most refined of all mineral oil basestock. It performs well in many regards and is highly stable. Lubricants formulated with this basestock are often synthetic or semisynthetic. Group III basestock is manufactured using the same hydro processing techniques as Group II basestock. The difference is that the processes are stepped up in order to yield a higher VI. Today’s Group III basestock performs as well or better than traditional synthetic oils. Group III fluids are only available up to about an ISO 68 grade.

  • Group IV or PAO (Polyalphaolefin) Basestock is a chemically engineered synthetic. It has a highly stable chemical composition and is increasingly present in synthetic and synthetic-blend products for industrial and vehicle applications. Newer PAO’s include improved mPAO (metallocene polyalphaolefin) bases.

  • Group V Basestock is usually blended with other basestock. It is used in small amounts as secondary basestock to impact lubricant properties. This basestock is capable of accommodating a wide variety of properties and custom packages. Because of this it provides the base ingredient for many additives. Group V base stocks are any synthetic fluid which does not fill any of the above 4 categories and would include Polyalkylene glycols (PAG'S), various esters, Alkylated Naphthalenes (AN), Silicones and others. Since group V's represent such a wide variety, it is difficult to make general statements about them. Many are used directly as base fluids, while others may be added as a co-basestock to other fluids.

At SA Performance, we blend Group III basestock from our proprietary formulas created by our STLE CLS® formulator. Group III fluids are severely hydrocracked/hydrotreated base fluids. As such, the levels of unsaturates and sulfur compounds are nearly eliminated. These extremely pure base fluids rival Group IV (PAO's) in performance in many areas. Group III performance, when formulated with an advanced additive package, provides fluid life equal or better than many other synthetics.

Call us at (904) 723-4946 or Email us for more information about the products and services provided by:

Simple Air Performance


Simple Air End Solutions

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