Base oil groups for coolants, quenchants explained

A primer on the different types of base oils as classified by the API and the refinery process used to produce the oils used in modern coolants, lubricants, and oil-based quenchants.


In this column, I will discuss the different groups of oil used in coolants, quenchants, and other oil-based products.


Many products that are used in a modern manufacturing facility use different types of oils. These could be oil-based quenchants, coolants, or lubricants. In each case, these oils start out as crude oils. Crude oil is composed of mixtures of different hydrocarbon compounds, such as paraffins, aromatics, naphthenes, and nitrogen and sulfur hetrocycles. Typical crude composition from different sources is provided in Table 1.

Base oils are the primary constituent in coolants, oil quenchants, and oils used for lubrication. The addition of additive packages creates a finished product that satisfies certain criteria for viscosity, flash point, lubricity, and thermal stability.

The American Petroleum Institute (API) has established a classification system to categorize base oils into different groups. This system, known as the API base oil classification, is based on the refining methods and the properties of the base oils. This system helps understand the performance characteristics and suitability of base oils for specific applications.

Table 1: Typical composition from different crude sources [1].

API Base Oil Classification

The API base oil classification system categorizes base oils into five main groups, with each group representing a different level of refining and performance characteristics. These groups are as follows.

Group I

Group I base oils are the least refined among the API base oil groups. They are produced through solvent refining or conventional refining processes and contain a significant quantity of impurities, including sulfur, nitrogen, and aromatics. Group I base oils typically
have lower viscosity indexes, lower oxidative stability, and lower saturate content compared to the higher groups. They have an amber to dark brown color and have a viscosity index (VI) of 90-105. These oils are commonly used in applications where low cost is a primary concern and where high-performance characteristics are not required. They find use in general-purpose lubricants and industrial applications.

Group I base oils are produced through conventional refining, also known as solvent refining. The process involves the following steps:

Solvent Extraction: In the first step, crude oil is treated with a solvent, such as furfural or phenol, to remove impurities, including aromatic compounds and contaminants. This solvent extraction process helps in reducing the levels of sulfur and nitrogen.

Hydro-finishing: The solvent-extracted oil is then subjected to hydro-finishing, which involves the use of hydrogen under high temperature and pressure. This step further reduces impurities and enhances the oil’s color and oxidative stability.

Dewaxing: If necessary, dewaxing can be performed to remove waxes and improve the oil’s low-temperature properties.

The resulting base oil is relatively less refined and contains higher levels of impurities compared to higher API groups. As a result, Group I base oils are typically used in less demanding applications.

Group II

Group II base oils are produced through hydro-processing, a more advanced refining method that removes impurities and unwanted components from the base oil. These oils have improved properties compared to Group I base oils. They have higher viscosity indexes, better oxidation stability, and lower levels of impurities such as sulfur. They are clear and colorless and have a VI above 100-120. Group II base oils are widely used in various automotive and industrial lubricant applications, including engine oils, hydraulic fluids, and gear oils.

Group III

Group III base oils are also produced through hydro-processing, but they undergo more severe refining than Group II base oils. This extensive refining process results in base oils with even higher purity, better oxidative stability, and improved viscosity characteristics. Like Group II, these oils are clear and colorless, but have a viscosity index above 120. Group III base oils are considered as synthetic base oils due to their enhanced properties. They are commonly used in high-performance engine oils, transmission fluids, and other applications where superior thermal and oxidative stability is required.

Gas-to-Liquid (GTL) base oils are classified as Group III oils. However, the creation of these oils is by taking a mixture of hydrogen and carbon monoxide (CO) from the partial oxidation of methane. Using a catalyst, the mixture of hydrogen and carbon monoxide (CO) (after removing impurities), is turned into the liquid hydrocarbon that resembles wax at room temperature. A final step of cracking and isomerization takes this wax-like liquid and turns it into an oil of the desired properties. These oils are water-white and offer a high flash point and exceptional thermal stability. The downsides are limited viscosity ranges and high cost.

Group II and III base oils (except for GTL oils) are produced through hydro-processing. The hydro-processing process involves the following steps:

Hydrocracking: In this step, the feedstock is subjected to hydrogenation in the presence of a catalyst. This process breaks down larger hydrocarbons, removes impurities, and saturates the molecules. It results in base oils with improved properties, including higher viscosity indexes and lower sulfur content.

Hydrotreating: After hydrocracking, the base oil undergoes hydrotreating to further remove impurities, such as sulfur and nitrogen compounds. This step enhances the oil’s oxidative stability and color.

Dewaxing: Dewaxing is performed if necessary to improve low-temperature properties.

The resulting base oils in Group II and III have significantly better properties compared to Group I base oils and are used in a wide range of automotive and industrial applications.

Group IV

Group IV base oils are fully synthetic and are manufactured using a process called polyalphaolefin (PAO) synthesis. These base oils have good properties at lower temperatures, and thermal stability at elevated temperatures. They have a narrow molecular weight distribution, which means that properties are stable and reproducible.

Group V

Group V base oils include all other base oils not classified under Groups I to IV. This category encompasses a wide range of base oils, including polyalkylene glycols (PAGs), esters, and other specialty fluids. Group V base oils are used in specific applications where their unique properties (such as biodegradability, lubricity, or high-temperature stability) are required. Examples of applications include refrigeration compressors, food-grade lubricants, and some industrial processes.


In this column, I have briefly described the different types of base oils as classified by the American Petroleum Institute and described the overall refinery process used to produce the different base oils used in modern coolants, lubricants, and oil-based quenchants.

Should you have any questions regarding this column, or suggestions for different topics, please contact the author or the editor. 


  1. US Department of Labor, “Petroleum Refining Process,” in OSHA Instruction TED 01-00-015, Washington D.C., US Department of Labor.