Understanding Synthetic Base Oils: A Closer Look
Base oils form the foundation of all lubricants, making them a critical component in ensuring optimal performance across various applications. Given the number of new visitors to our website and the increasing enquiries we receive regarding base oils, we felt it appropriate to revisit this topic—particularly synthetic base oils.
API Classification of Base Oils
The American Petroleum Institute (API) classifies hydrocarbon-based oils into five distinct groups, based on refining methods and key properties such as viscosity index (VI), saturates, and sulphur content.
Group I: Conventional Mineral Oils
Group I base oils are the least refined and are produced through solvent refining—a process that removes undesirable compounds and impurities. These oils typically have a viscosity index of around 100 and are straw to light brown in colour. Although still in use, they are being increasingly replaced by higher-quality alternatives.
Group II: Highly Refined Mineral Oils
Group II base oils undergo hydrocracking, a process that removes most unstable aromatic hydrocarbons and sulphur. As a result, these oils are clearer, more stable, and have improved performance characteristics. They typically feature a viscosity index of around 110, with some high-quality versions exceeding 115. Due to their enhanced properties and competitive pricing, they are widely used in modern lubricants.
Group III: The Highest Quality Mineral Oils
Group III base oils are refined through hydrocracking, hydroisomerisation, and hydrotreating, making them significantly purer than Group I and II. They boast a viscosity index greater than 120 and, while derived from mineral sources, their characteristics closely resemble those of synthetic Group IV oils.
Group IV: True Synthetic Base Oils (PAOs)
Group IV oils consist of chemically engineered synthetic hydrocarbons known as polyalphaolefins (PAOs). These oils contain no unsaturated hydrocarbons, sulphur, nitrogen components, or waxes, making them highly stable with a viscosity index typically above 130. PAOs offer superior high- and low-temperature performance, excellent oxidation stability, and compatibility with mineral oils, making them the most common synthetic base oils in automotive and industrial lubricants.
Group V: Speciality Base Oils
Any base oil not classified within Groups I–IV falls under Group V. This diverse category includes esters, polyglycols, naphthenes, polybutenes, silicones, and biolubricants. These oils are often blended with others to enhance performance characteristics. For instance, PAO-based compressor oils are frequently combined with polyolesters to improve thermal stability and detergency.
The API categorizes lubricating base oils according to their chemical and physical properties as shown below:
| PROPERTY | Gr 1 | Gr 2 | Gr 3 | Gr 4 | Gr 5 |
| Saturates | <90% and/or | >90% and | >90% and | P A | O T |
| Sulphur | >0.03% and | <0.03% and | <0.03% and | O | H E |
| Viscosity Index | 80 – 120 | 80 – 120 | >120 | >125 | R |
The Debate Over Synthetic Lubricants
Rewinding to the late 1990s, a landmark legal battle unfolded in the USA between Mobil and Castrol over the definition of synthetic oils. Mobil contested Castrol’s claim that its Group III oils were synthetic. However, in 1999, Castrol successfully argued that their extensive hydroprocessing modifications qualified Group III oils as synthetic. As a result, the API removed ‘synthetic’ from its classification system, turning it into a marketing term rather than a scientifically measurable category.
Terminology in Today’s Market
Currently, hydrocarbon base oils are typically described as follows:
Mineral Oils: Group I and II oils are referred to as mineral base oils or highly refined base stocks.
Semi-Synthetic Oils: A blend of Group I and/or II oils with Group III and/or IV. These are also labelled as part-synthetic, synthetic-based, or synthetic technology oils.
Synthetic Oils: Following the 1999 ruling, Group III oils are commonly marketed as synthetic.
Full Synthetic Oils: This term is usually reserved for Group IV polyalphaolefin (PAO) base oils.
Group V oils are generally identified by their chemical names, such as polyolester, alkylbenzene, polyalkylene glycol, and polyisobutylene, sometimes prefixed with ‘synthetic’.
Conclusion
Understanding base oils and their classifications is crucial when selecting the right lubricant for your application. With the evolution of refining processes and changing industry definitions, synthetic lubricants continue to be a key area of discussion. If you have any questions about base oils or need assistance in choosing the right lubricant, feel free to reach out to our team—we’re happy to help!