Understanding Base Oil Categories

Before lubricant is blended with additives, it constitutes of just the base oil; in case of grease, the base oil is blended with a thickener. Base oils for industrial lubricants belong to two different categories – synthetics and mineral oil, and they are present in all lubricants. Selecting the ideal base oil for any given application is a critical decision. So it is important to understand the complexity of your base oil formulations to avoid any confusion in selecting them. This article will provide you with all the information you need for selecting your base oil.

Sources of Base Oils

Industrial lubricants are categorized depending on the type of base oil they contain; the most prominent ones are –

1. Mineral oil – This type of oil is refined from crude oil and its quality depends on the quality of the process used for refining the material. 

2. Synthetic oil – Synthetic oils get unique properties that are required for their intended purpose through man-made synthesizing processes.

3. Vegetable oils – Few lubricants that are utilized for renewable and environmental purposes belong to this category. They are derived from plant oils.

Base oil Characteristics

The characteristics of base oils determine their function and reliability. A superior mineral oil requires the best refining processes while the objective of synthetic oil is to achieve properties that are not achievable in mineral oils. Each base oil serves a different purpose, not every function can be ideally served using a single type of base oil. Research shows that 55% of plants use both – mineral based and synthetic oils in their lubrication program.

Some important properties in your base oil include thermal stability, viscosity limitation, viscosity index, volatility, oxidation, pour point, hydraulic stability and aniline point (the measure of solvency with other materials and additives)


Extending the service life of your equipment in extreme conditions requires custom-made solutions which can only be provided by synthetic oils. If your equipment frequently encounters high operating temperatures and cold startup conditions, it requires oxidative and thermal stability properties. Synthetic oils like PAOs are ideal for such applications rather than mineral oils. PAOs also show better hydraulic stability and demulsibility hence they manage water contamination better than mineral oil as well.

Mineral oils, on the other hand, are more widely preferred due to their reasonable service capabilities and relatively lower costs. They are widely used in industrial and automotive sector; with over 90 percent usage, mineral oil has solidified its position in the market for most applications.

Groups of base oils

The refining process of mineral oils has improved greatly in the 20th century. The American Petroleum Institute (API) divided all base oils into five separate groups in the early 1990s. The first 3 groups were dedicated to mineral oils and the 2 remaining groups contained synthetic oils predominantly. 

Group 1, 2 and 3 are all mineral oils, with chronologically increasing refining process severity. Base oils under Group 1 are made using solvent-refining or solvent-extraction technology.

Base oils under Group 2 are produced by hydrotreating or hydrogenation. It is a process that uses hydrogen gas to refine oils, but they are better for converting components like aromatics that are undesirable, into hydrocarbon structures that are desirable.

Base oils in Group 3 are produced in a similar fashion as Group 2, the only difference is that the process of hydrogenation or hydrotreating in group 3 is coupled with high pressures and high temperatures. This leads to convert all the undesirable components in the oil into desirable hydrocarbon structures.  

When you compare these groups of mineral oil bases, you will see that highly refined oils provide greater benefits in terms of thermal stability, oxidation stability, pour point, operating temperature, and viscosity index. But the oil starts to show some key weaknesses, affecting additive solubility and oils biodegradability.

Group 4 contains only one type of synthetic oil called polyalphaolefin (PAO), these synthetically created hydrocarbons are the most widely used base oils form the synthetic category. They are created using the polymerization process that gives an olefinic tail by the introduction of ethylene gas. This process makes the chemical structure of PAOs quite similar to that of highly refined mineral oils form Group 3. While PAOs lack in lubricity, additive solubility, film strength and seal shrinkage, they have powerful advantages such as superior oxidation stability, higher viscosity index, lower volatility and excellent performance in high and low temperatures. POWs are employed widely for lubrication; they are especially preferred when the equipment is expected to reach higher temperatures.

Group 5 contains all the remaining oil bases which are mostly synthetics. Most common oils that belong to this group are polyolesters, diesters, polyalkylene glycols, silicones and, phosphate esters.

Selecting base oils

Choosing a base oil requires you to make tradeoffs for the lubrication properties you require for the required application. Consider viscosity for example – when it is high you get strong film strength and when it’s low you get optimal energy consumption and low-temperature fluidity. You may in some cases prefer to have a balance between these properties to avoid compromising on either side.

It is very important to understand the base oil options available to you and their respective advantages and disadvantages. Selecting justifiably more expensive base oil is always better than risking an equipment failure. Always optimize your base oil selection to minimize the risk of equipment failure.
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The information contained herein has been compiled from sources deemed reliable and it is accurate to the best of our knowledge and belief. However, Castrol cannot guarantee its accuracy, completeness, and validity and cannot be held liable for any errors or omissions, as the results change depending on the working condition/environment.  Changes are periodically made to this information and may be made at any time.
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