Lubricant Basics – Base Stocks

What are Base Stocks?

Base stocks also known as base oils, make up most of the volume in lubricants, accounting from 75%-90% depending on the application of the formulation. Base Stocks that are used in the manufacturing of lubricants are normally of mineral (petroleum) or synthetic origin. 


Base Oil Tanks 2

Synthetic vs Mineral

The property of the base stocks can vary from one another and will impact the lubricant performance. With the addition of additives, the base stock now enhanced will provide additional benefits to the lubricant, this including cleaning and wear protection.

Due to the synthetic processing of synthetic base stocks, they can give a precise property required i.e. their pour points are considerably lower. This makes them more valuable blending components when compounding oils for extreme service is either high or low temperatures. The main disadvantage of synthetics is that they are more expensive than mineral oils. However, with mineral oils phasing out, the reverse is true.


Base Oil Categorization

As base stocks are often used interchangeably, the American Petroleum Institute (API) have categorised it into five classifications:

Group I, II, III

These group of base stocks includes all paraffinic base stocks (mineral stock) from petroleum crude oil and are differentiated by the severity of the manufacturing process. This increases from Group I to Group II to Group III depends on the distillation severity.

Group III

Also includes synthetic base stocks for example liquid hydrocarbons derived from natural gas, coal or biomass.

Group IV

This group of synthetic base stocks are dedicated to polyphaolefins (PAO), which are derived from ethylene. Polyphaolefins are most widely used synthetic lubricants in the US and Europe. They have good stability but are limited to the ability to dissolve some additives which are easily overcome by adding a small amount of ester.

Group V

Mainly includes all the other types of base stocks such as synthesised esters of polyalkylene glycol (PAG) and also other napthenics.

GroupViscosity IndexSaturates SulphurOther
I80 ≤ x < 120< 90%and/or> 0.03% 
II80 ≤ x < 120≥ 90%and≤ 0.03% 
III≥ 120≥ 90%and≤ 0.03% 
IV    PAO synthetic lubricants

Other base stocks not included in Group I, II, III

As you move down the group from Group I to Group IV, key lubricants performance properties of base stocks, such as oxidation, traction, volatility, and low temperature performance tend to improve.

You may also find additional names are used in the base stock market to describe certain characteristics, such as kinematic viscosity, viscosity index and manufacturing process.

KV 100 Value

This used to differentiate between the ‘light grades’, ‘heavy grades’ and ‘brightstock’ and is associated with base stock numeral symbol

For example: PAO 4 is a Group IV light grade oil with KV100 of 4cSt


This symbol is based upon the manufacturing process. SN stands for ‘Solvent Neutral’ and used in Group I base stocks.

For example:  SN150 means it is a Group I grade base stock with a KV100 of 5cSt


This term is used to discriminate synthesised base stocks such as GTL or PAO.


Base stock with a high viscosity index are market with a ‘+’ symbol.

For example: Group II+ means a VI greater than 110, Group III+ means a VI greater than 130

Base Stock Usages

When selecting a base oil, the first criteria to consider is the required performance. For high performance and top tier application such as SAE 0W-X or 5W-X oils would require a static and dynamic behaviour at high and low temperatures. This lubricant would need high quality synthetic base stocks such as Group III and PAOs. For lower performances, there is a variety of base stocks to choose from.

Generally, this is influenced by the formulation costs which can vary from region to region based on availability.

Base Oil Storage tanks