Base Oil Groups: A Basic Guide

How many groups of base oils are there and how do they differ?

The American Petroleum Institute (API) classifies base oils into 5 separate categories. Those 5 categories are Group I, Group II, Group III, Group IV, and Group V.  Groups I, II, and III are refined from petroleum crude oil.  Group IV base oils are considered “full synthetic” and carry the fancy name of polyalphaolefin (PAO).  All other base oils not included in Groups I through IV are classified asr Group V. 

Group I — Slightly refined

Group I base oils are the least refined base oil.  They are classified as less than 90 percent saturates and greater than 0.03% sulfur.  These oils go through the least amount of refining and purification therefore are the cheapest base oils on the market.  If your engine is consuming high levels of oil, most likely it is burning a Group I oil.

Group II — Better in cold, resist oxidation

Group II base oils are more refined than Group I.  They go through a process called hydrocracking which essentially breaks up the larger molecules in the oil into smaller molecules.  As a result, Group II base oils will exhibit better performance is colder temperatures as well as better antioxidation properties.  Their appearance will be clearer than Group I base oils. 

Group III — Viscosities 120+

Group III base oils are known as “brightstock” and are even more refined that Group II.  They are also classified as less than 90 percent saturates and greater than 0.03% sulfur. These oils are severely hydrocracked (even higher heat and pressure). This process creates an even purer base oil with a viscosity index of 120 and above.

Group IV — Synthetics

Group IV base oils are chemically referred to as polyalphaolefins or PAO for short.  These synthetic base oils are essentially made in a laboratory and are derived from ethylene gas.  They exhibit a viscosity index of above 140 and have a much broader operating temperature range that are great in extreme cold conditions and high heat applications.

Group V — Extreme performers

Group V base oils are classified as all other base oils that are not considered in the above categories. These include oils such as silicone, polyolester, biolubes, Polyalkylene glycol (PAG), etc.  Esters fit in the Group V category which are known to enhance the properties of existing oils.  These enhancements include excellent detergency, superior high temperature and hydrolytic stability, high viscosity index and shear stability, as well as increasing the hours an oil can last.

The Alpha Performance Booster is classified in this Group V category.

Contact Alpha Lubricants for more information

Alpha Lubricants offers a variety of performance boosting fuel additives. We are ready to take your questions and offer expert assistance with your engine oil questions. You can contact us on our website or call us at 724-941-4454

Alpha Lubricant’s mission

As a leader in performance and protection lubricants, it is our mission and responsibility to help our customers in achieving operational excellence for all their vehicles and machinery. Our goal is to greatly increase your engine life expectancy, performance and fuel efficiency while simultaneously lowering operation and maintenance costs to an absolute minimum.

About the author

Jim Silvestros, Jr learned the oil business from his father Jim Silvestros, Sr. He has been CEO of Alpha Lubricants since 2010. He lives in Pittsburgh with his wife and two children.