Post-refined oil plays a large role in the performance of today’s engines, so it is important to understand the properties and differences from one type of base oil to the next.
Groups of base oils
As a basic primer, base oils are classified into five different categories as specified by the American Petroleum Institute (API). Before the base oils are additized – where detergents, dispersants, viscosity improvers and friction modifiers are added in the post-refinery process – they are grouped based on the refining process, in addition to the oil’s sulfur levels, saturate levels and viscosity index (VI).
The VI is a measure of the change in viscosity from set temperature points of 40 degrees C and 100 degrees C. Although viscosity for engine oils is usually measured in centistokes (cSt), the VI contains no units and is an arbitrary number. A higher VI value means that an oil is less affected by temperature changes and maintains its viscosity. The lower the VI, the more an oil is affected by temperature change. It should also be noted that when comparing oils, the Viscosity Index is only relevant when comparing oils of the same SAE grade. For example, only compare the VI of a 40 weight to another 40 weight, not a 30 weight to 40 weight.
Group I
Group I base oils have the highest amount of sulfur or the lowest amounts of saturates with a VI range of 80 to 120. The earliest refining process was done through acid refinement or clay refinement. Today, however, these lubricants are typically solvent-refined.
While they provide excellent solvency for additives, Group I base oils have poor resistance to oxidation, sludge formation and thermal breakdown. Due to their high levels of sulfur, Group I base oils do not meet the API CK-4 or FA-4 specifications and are no longer used as engine oils. When Group I base oils were used as engine oils, their condition needed to be monitored very closely as they fell out of spec rapidly which lead to damaging wear inside of the engine.
Group II
Group II base oils have a sulfur level of less than 3 percent, saturates above 90 percent and a VI of 80 to 120. Group II base oils are typically hydrotreated (lightly hydrocracked) and catalytically de-waxed.
Group II oils are referred to as conventional engine oils.
The higher saturates levels in Group II base oils provided more oxidation and thermal stability than Group I base oils, allowing them to remain in spec for longer periods of time before breaking down and needing to be removed from an engine.
Group III
Group III base oils have similar sulfur and saturate levels as Group II, but have a VI greater than 120. They are refined by a severe hydrocracking process, which uses more pressure and heat, and an advanced catalytic dewaxing process that converts the oxidatively unstable olefins (alkenes) into the more stable paraffins.
Group III base oils are typically, though incorrectly, referred to as “synthetics” due to hydrocarbons being synthesized during the refining process.
Group III base oils have the highest oxidation and thermal stability among Group I, II and III base oils and are becoming more prevalent in the industry because of their ability to increase the overall efficiencies of today’s engines.
Group IV
Group IV base oils are synthetic polyalphaolefins, or PAOs, for short. PAOs have superior oxidation, volatility and high-temperature/high-shear characteristics as compared to Group I through Group III base oils.
Group IV base oils are pure or true synthetics.
Group IV base oils are much better suited to both extreme cold and high heat conditions, and as such, have a broader operating temperature range. Although most oil manufacturers stop at Group III base oils, there are companies like Hot Shot’s Secret who manufacture a full line of oils including the Group IV PAOs. Hot Shot’s Secret Blue Diamond Oil is a 100-percent PAO formula, and is infused with a patented friction reducing nano technology.
Group V
Group V base oils is a catch-all group of synthetic oils that do not fit into any of the other four categories. Some of the examples for Group V base oils include polyalkylenes (PAGs), alkylated benzenes, polyolesters (POEs), biolubes (plant-based), silicone derivatives and phosphate esters.
In other words, if it is a synthetic lubricant that is not a PAO, it belongs in Group V. In most cases, Group V oils are not used alone as a lubricant, but rather enhance the lubricating qualities of a Group I though IV base oil.
Alternative base oil
One newer chemistry not widely talked about, is Gas to Liquid, or GTL. These base oils are differentiated from the others because they are produced from natural gas rather than crude oil. GTL base oils have qualities that are close to the PAO synthetics and are often described as “Group III+”.
Conclusion
How do fleets know which base oil(s) the vehicle engine oil is comprised of? When using a conventional oil, most of the time it is Group II. Semi-synthetic base oils are usually a mixture of Groups II and III. There is some confusion in the industry about the nomenclature of synthetics. As a general rule, for the most part, if the bottle does not designate the oil as a PAO true synthetic, which is a Group IV, the “synthetic” engine oil would be categorized as a Group III.
It is important to remember that base oil quality increases from Group I through Group IV. Most of the oils being manufactured today fall between the Group I and Group III levels, with the exception of a few advanced companies producing oils up to the Group IV level. Group V oils are a specialty category for base oils that don’t fit into any other classification, so fleets should check the specifications to see if a Group V base oil is correct for each application.
Chris Gabrelcik is the president and owner of Lubrication Specialties, Inc. with certifications both as Lubrication Specialist (CLS) and Oil Management Analysis (OMA). Beginning with formulations for industry machinery, Gabrelcik has directed the company to its current role as a global leader in the development of fuel and oil additives for multiple markets including the automotive and heavy duty transportation aftermarket.
