Base oil Groups, Types and Classifications

According to API (American Petroleum Institute), a base stock is a lubricant component that is produced by a single manufacturer usually defined by its viscosity grade and group. API base stock groups were first defined to develop interchange guidelines in engine oils. Now a days API base stock grouping is a framework for establishing base oil interchange guidelines for most lubricants.

Base Oil vs Base Stock

A base oil and base stocks are two different terms but used  interchangeably.[1].  Base oil  is a mixture of one or more base stocks. On the other hand, a base stock is a single component from a manufacturer usually defined by its viscosity grade and API group. We blend or mix base stocks to make the finished products like lubricants and greases. In many regions, term base oil and base stock are used interchangeably.

API Base Oil Classifications Chart (Table)

API Group Viscosity index Saturates Sulfur
I ≥80 – <120 <90% and/or >0.03%
II ≥80 – <120 ≥90% and ≤0.03%
III ≥120 ≥90% and ≤0.03%
IV Polyalphaolefins
V All other products not meeting the requirements of the first four groups

API Group I Base Stock (conventional mineral base oils)

Group I base stocks are paraffinic mineral oil, solvent refined (solvent neutrals SN) have less saturates and more sulfur. “API Group I” base stocks have more than 0.03% sulfur and or less than 90% saturates. The viscosity index is in the range of 80 to 120. The main difference between Group I and Group II base stocks is sulfur content and saturates. Demand for Group I base oils are declining, but still, they are the largest single category in the global lubricants markets.

API Group II Base Stock

Group II base stocks are highly refined paraffinic mineral oil, having less than 0.03% sulfur and more than 90% saturates. API Group II base stocks are solvent refined and hydro-processed or hydrocracked. Hydro-processing and hydrocracking are some of the modern manufacturing methods employed in the manufacturing of Group II. The viscosity index for both Group I and Group II is in the range from 80 to 120. Typically, most manufacturers offer Group II base stock with a higher viscosity index of 100 or higher. Due to desired characteristics and strict standards, the demand for Group II base oils has increased.

API Group III Base Stock

Group III base oils are considered synthesized hydrocarbons with more than 120 viscosity index, less than 0.03 sulfur, and more than 90% (usually 99%) saturates. High viscosity index is a desired property of base oils. Group III base oils are severely hydrocracked to restructured the hydrocarbon molecules. Sever hydrocracking, Isomerization, and Hydro-finishing make the product very different from the actual feedstock. API group III base stocks are purer, cleaner, and water white. Due to the current trend towards low viscosity lubricants, Group III base stocks are becoming more prevalent.

API Group IV Base Stock

PAO or polyalphaolefins are API category Group IV synthesized base oils. Polyalphaolefin (PAO) is a synthetic lubricant manufactured through a chemical process that originates from ethylene.[2]. PAOs are free from sulfur, nitrogen, and other waxy components. Hence have a very high viscosity index, excellent oxidation stability, excellent low-temperature flow characteristics, and excellent pour-point.

API Group V Base Stock

Everything else not in the above groups is in Group V. Hence, all other base stocks not included in Group I-IV are in the API Group V base oil category. For example, pale oil, many vegetable oils, some non-PAO synthetic oils, organic esters, and polyalkylene glycols are in the Group V category.

What is base stock? 

A lubricant component produced by a single manufacturer is usually defined by its viscosity and API group.

What is base oil?

Base oil is a mixture of one or more base stocks. Base stocks are blended or mixed to make the finished products like lubricants and greases.

What is the base stock slate?

Base stock slate is components (base stock) from the same manufacture having the same API group but different viscosity. Hence, the base stock slate is technically interchangeable. For example, from the same manufacturer, Group I SN150, Group 1 SN 500, and Group 1 SN600 have different viscosity but are interchangeable and called base stock slate.


[1] API base oil interchange guidelines

[2] PAO