Base oil

What Is Base Oil?

Base oil is the primary ingredient in lubricants, forming up to 70–99% of the final product, depending on the application. It is derived from refined crude oil (mineral base oils) or synthesized from chemical compounds (synthetic base oils). The base oil determines many of the lubricant’s properties, including viscosity, oxidation stability, volatility, and performance under pressure.

Base oils are used in a wide range of applications including:

  • Engine oils

  • Hydraulic fluids

  • Gear oils

  • Transmission fluids

  • Greases

  • Industrial lubricants

Base Oil Categories (API Groups)

The American Petroleum Institute (API) classifies base oils into five groups based on sulfur content, viscosity index (VI), and level of refining:

Group Type Sulfur Content Saturates Viscosity Index (VI) Description
I Mineral >0.03% <90% 80–120 Solvent refined, lower performance
II Mineral <0.03% >90% 80–120 Hydrotreated, better oxidation stability
III Mineral (Highly Refined) <0.03% >90% >120 High VI, used in synthetic-like products
IV Synthetic (PAO) 0% 100% >125 Polyalphaolefins (PAOs), excellent performance
V Others (Ester, PAG, etc.) — — Varies For specialty applications

Key Properties of Base Oils

  • 🔸 Viscosity: Resistance to flow; critical for lubrication performance at different temperatures.

  • 🔸 Oxidation Stability: Resistance to breakdown from heat and oxygen.

  • 🔸 Volatility: Tendency to evaporate at high temperatures.

  • 🔸 Pour Point: The lowest temperature at which the oil can still flow.

  • 🔸 Thermal Stability: Ability to maintain structure under high temperatures.

  • 🔸 Compatibility: Must work well with additives and seals in machinery.

Applications of Base Oil

  • Automotive Lubricants: Engine and transmission oils rely on specific base oil types for optimal protection and fuel efficiency.

  • Industrial Machinery: Compressors, turbines, and gearboxes require stable base oils with high oxidation resistance.

  • Hydraulic Systems: Depend on base oils with excellent low-temperature fluidity and anti-wear characteristics.

  • Metalworking & Processing: Used in forming fluids, cutting oils, and rust preventives.

Mineral vs Synthetic Base Oils

Feature Mineral Base Oils Synthetic Base Oils
Cost Lower Higher
Temperature Range Moderate Wide (extreme hot/cold)
Oxidation Stability Moderate Excellent
Viscosity Index Lower Higher
Applications General High-performance, severe conditions

Conclusion

Base oil is the cornerstone of all lubricants, directly impacting their effectiveness, longevity, and performance. With a wide variety of types available — from basic Group I oils to advanced Group IV synthetics — selecting the right base oil is crucial for ensuring the efficiency, protection, and reliability of engines and industrial systems.