Groups and Categories
- Group I Base Oils (paraffinic)
- Group II Base Oils (hydrocracking)
- Group III Base Oils (hydrotreating)
- Group IV Base Oils – Polyalphaolefins (PAOs)
- Group V Base Oils – (PAGs & Esters)
Group I Base Oils
Production & Composition
Derived from paraffinic crude oil using solvent extraction and dewaxing.
- <90% saturates
- >0.03% sulfur
- Viscosity Index (VI): 80–119
- Typically darker in color (white to dark brown)
Properties
- Cost-effective but lower in oxidation stability
- Higher viscosity and moderate flash points
- Lower purity than higher-group oils
Applications
- General-purpose lubricants
- Automotive engine oils (older specifications)
- Industrial oils, hydraulic fluids, and metalworking fluids
- Used where cost-efficiency is prioritized over extreme performance
Group II Base Oils
Production & Composition
Produced by hydrocracking—a more intense refining method.
- >90% saturates
- <0.03% sulfur
- VI: 80–120
- Water-white in color and clearer than Group I
Properties
- Improved oxidation stability over Group I
- Lower volatility, better viscosity control
- Enhanced corrosion protection
- More thermally stable due to high saturation levels
Applications
- Engine oils (API SN, SP, etc.)
- Transmission fluids
- Industrial hydraulic oils
- Replacing Group I in many applications due to similar cost and better performance
Group III Base Oils
Production & Composition
Created via hydrocracking, hydroisomerization, and hydrotreating.
- >90% saturates
- <0.03% sulfur
- VI >120
- Often classified as “synthetic” or VHVI (Very High Viscosity Index) oils
Properties
- Excellent thermal and oxidation stability
- Superior low-temperature performance (low pour points)
- Higher VI for stable viscosity across a wide temperature range
Applications
- High-performance automotive lubricants
- Gear oils, driveline fluids, and industrial gear lubricants
- Ideal for severe service applications (hot and cold extremes)
Group IV Base Oils – Polyalphaolefins (PAOs)
Production & Composition
- True synthetic oils, produced by polymerizing alpha-olefins (usually 1-decene)
- Chemically engineered for high purity and consistent molecular structure
Properties
- Very high VI (~130) and excellent stability at both high and low temperatures
- Low pour points, high flash points
- Non-toxic and low in volatility
- Outstanding oxidation resistance and low friction characteristics
Applications
- Aviation lubricants, jet engine oils
- High-performance engine oils, gear and transmission fluids
- Compressor oils, hydraulic systems, and severe industrial environments
Variants
- Dimer PAO (C10–C12)
- MPAO (metallocene-based): used in super-high viscosity applications
Group V Base Oils
Production & Composition
- Esters (e.g., polyol esters)
- Polyalkylene Glycols (PAGs)
- Silicones
- Phosphate esters
- Bio-lubes (plant-based oils)
Properties
- Wide-ranging properties depending on chemistry
- Esters: excellent high-temperature performance and detergency
- PAGs: water-soluble, excellent film strength, used in fire-resistant fluids
Applications
- Compressor oils, refrigeration lubricants
- Biodegradable lubricants
- Specialty industrial fluids
- Often blended with PAO or Group III to enhance film strength or detergency
Comparison Summary
| API Group | Sulfur Content | Saturates | VI | Base Oil Type | Typical Use |
|---|---|---|---|---|---|
| Group I | >0.03% | <90% | 80–119 | Solvent refined mineral oil | General purpose |
| Group II | <0.03% | >90% | 80–120 | Hydrocracked mineral oil | Engine & industrial oils |
| Group III | <0.03% | >90% | >120 | Severely hydrocracked | Synthetic-performance oils |
| Group IV | 0% | 100% | ~130 | PAO (synthetic) | Aviation, premium lubricants |
| Group V | Varies | Varies | Varies | Esters, PAGs, etc. | Specialty & blended applications |
Base oils, form the fundamental building blocks of lubricants. Their classification into Groups I–V reflects their refining processes, chemical structure, and performance capabilities. Group I offers affordability for basic applications, while Groups II and III strike a balance between cost and performance. Group IV (PAO) and Group V (specialty synthetics) offer premium performance for the most demanding environments. Understanding these base oil categories is crucial for formulating effective lubricants, maximizing equipment protection, and meeting evolving environmental and performance regulations.