API Base Oil Groups (Group I – Group V): Clear Technical Overview and Uses
Base oils form the core of all lubricants. Because of this, their structure and purity directly affect oil performance, lifetime, and application areas. The American Petroleum Institute (API) groups base oils into five main categories. These groups depend on saturation level, sulfur content, and viscosity index.
Below is a clear and practical explanation of Group I to Group V base oils, including technical details and where they are used.
Group I Base Oils
Overview
Group I base oils come from solvent refining, which is the oldest refining method. As a result, these oils contain more sulfur and aromatic compounds. However, they still remain widely used due to their lower cost and good solvency.
API Definition
Base stocks contain less than 90% saturated hydrocarbons, more than 0.03% sulfur, and a viscosity index between 80 and 120.
Key Features
- Higher sulfur content
- Good additive solubility
- Darker color
- Moderate resistance to oxidation
Typical Specifications
| Parameter | Typical Value |
|---|---|
| Saturates | < 90% |
| Sulfur | > 0.03% |
| Viscosity Index | 80 – 120 |
| Color | Dark / amber |
| Oxidation Stability | Medium |
Group II Base Oils
Overview
Group II base oils are produced using hydrocracking and hydrotreating. Therefore, most impurities are removed. As a result, these oils are cleaner, clearer, and more stable than Group I oils.
API Definition
Base stocks are 90% or more saturated, contain 0.03% sulfur or less, and have a viscosity index between 80 and 120.
Key Features
- Low sulphur
- Light color
- Better heat resistance
- Longer oil life
Typical Specifications
| Parameter | Typical Value |
|---|---|
| Saturates | ≥ 90% |
| Sulfur | ≤ 0.03% |
| Viscosity Index | 80 – 120 |
| Color | Clear |
| Oxidation Stability | High |
Group III Base Oils
Overview
Group III base oils go through advanced hydroprocessing. Because of this, they reach very high purity levels. Although they come from crude oil, many markets treat them as synthetic oils.
API Definition
Base stocks are 90% or more saturated, contain 0.03% sulfur or less, and have a viscosity index of 120 or higher.
Key Features
- Very clean structure
- Low evaporation
- Strong heat resistance
- Stable performance
Typical Specifications
| Parameter | Typical Value |
|---|---|
| Saturates | ≥ 90% |
| Sulphur | ≤ 0.03% |
| Viscosity Index | ≥ 120 |
| Volatility | Low |
| Oxidation Stability | Very High |
Group IV Base Oils (PAO)
Overview
Group IV base oils are fully synthetic and made from polyalphaolefins (PAO). Therefore, they perform well in extreme conditions where mineral oils fail.
Key Features
- Very low pour point
- Strong resistance to heat and oxidation
- Long service life
- Stable in hot and cold climates
Typical Specifications
| Parameter | Typical Value |
|---|---|
| Viscosity Index | 125 – 200 |
| Pour Point | Very low |
| Oxidation Stability | Excellent |
| Volatility | Very low |
As a result, PAO oils are ideal for Northern Europe’s cold weather and Middle East high temperatures.
Group V Base Oils
Overview
Group V includes all base oils not covered in Groups I–IV. In practice, these oils support performance rather than act alone.
Common Types
- Esters
- Alkylated naphthalenes
- PAG oils
Key Features
- Strong lubrication
- Good cleaning ability
- Better seal compatibility
- Often used in blends
Typical Specifications
| Parameter | Typical Value |
|---|---|
| Structure | Non-mineral |
| Lubricity | High |
| Thermal Stability | High |
| Solvency | Excellent |
Base Oil Groups and Their Uses
| Group | Main Uses |
|---|---|
| Group I | Marine oils, industrial oils, gear oils |
| Group II | Engine oils, hydraulic oils, compressors |
| Group III | Passenger car oils, low-emission engines |
| Group IV | High-performance engines, aviation oils |
| Group V | Specialty oils, refrigeration, racing oils |
Conclusion
Each API base oil group serves a specific purpose. Therefore, choosing the correct group is critical for performance and cost control. While Group I oils remain useful in industry, Groups II and III dominate modern engines. Meanwhile, Groups IV and V support high-performance and special applications.
As a result, understanding base oil groups helps producers and users make better technical and commercial decisions.