Base Oil Groups Explained

Understand the foundation of every lubricant — and why base oils matter more than you think.

Base oils are the fundamental building blocks of all lubricants. Whether you’re pouring conventional oil into a lawnmower or synthetic oil into a high-performance engine, what you’re really choosing is a specific type of base oil — and the additives that enhance it. The American Petroleum Institute (API) classifies base oils into five main groups, each with different levels of refining, purity, and performance.

In this guide, we’ll break down the API base oil classification system, compare Group III vs Group IV synthetic base oils, and explain how each group impacts lubricant performance, longevity, and cost.

What Are Base Oils?

Base oils are the primary ingredient in lubricants, typically making up 70–90% of the final formula by volume. They provide the essential lubricating properties — reducing friction, carrying heat away from components, and suspending contaminants until they’re filtered out.

There are two main sources of base oils:

Mineral base oils, which are refined from crude oil.
Synthetic base oils, which are engineered or chemically modified for superior performance.

The API categorizes base oils into five groups (I to V), based on how they’re made and how they perform. Understanding these base oil groups helps consumers, mechanics, and fleet managers make smarter lubricant choices.

API Base Oil Groups (I–V)

The API base oil classification system is based on three main properties:

Saturates content (a measure of chemical stability)
Sulfur content
Viscosity Index (VI) — how stable the oil is across temperature ranges

Solvent-Refined Mineral Oils

Group I

Source: Crude oil
VI: 80–119
Saturates: <90%
Sulfur: >0.03%

Group I base oils are the least refined. They’re made using solvent extraction and hydrofinishing, which leaves behind more impurities and a lower viscosity index. They are cost-effective and commonly used in industrial lubricants and some older engine oils.

✅ Common in: Basic motor oils, gear oils, industrial oils

Hydroprocessed Mineral Oils

Group II

Source: Crude oil
VI: 80–119
Saturates: ≥90%
Sulfur: ≤0.03%

Group II oils are more refined than Group I. They have better chemical stability and lower sulfur content, making them suitable for many modern motor oils. Most conventional oils on the market today use Group II base stocks.

✅ Common in: Modern conventional motor oils, OEM factory-fill oils

Highly Refined Mineral Oils (Hydrocracked)

Group III

Source: Crude oil
VI: ≥120
Saturates: ≥90%
Sulfur: ≤0.03%

Group III oils are processed using hydrocracking, which creates a more uniform molecular structure and increases performance. Despite being derived from crude oil, their performance characteristics rival synthetic oils — so much so that they can legally be labeled as synthetic in the U.S., Canada, and many other regions.

✅ Common in: Synthetic-blend and full-synthetic motor oils

Note: This group is at the heart of the “synthetic vs. real synthetic” debate (see next section).

Polyalphaolefin (PAO) Synthetic Oils

Group IV

Source: Chemically synthesized from ethylene
VI: ≥125
Saturates: 100% (very pure)
Sulfur: 0%
Group IV base oils are true synthetics, built molecule-by-molecule in a lab. PAOs are exceptionally stable, offer excellent low- and high-temperature performance, and are highly resistant to oxidation and thermal breakdown.

✅ Common in: Premium full-synthetic motor oils, racing oils, aerospace lubricants

All Other Base Oils

Group V

Source: Various (esters, PAGs, alkylated naphthalenes, etc.)
Performance: Varies by chemistry

Group V is a catch-all for base oils that don’t fit into the other four groups. These are typically specialty synthetics, often used to improve seal conditioning, additive solubility, or extreme performance characteristics. Group V oils are rarely used alone — they’re typically blended with Group III or IV base oils.

✅ Common in: Ester-based racing oils, compressor lubricants, high-temp fluids

The Debate

Group III vs Group IV

One of the most hotly debated topics in lubricant chemistry is whether Group III oils should be considered “synthetic.” Here’s why it matters:

The Legal Definition

In North America, the term synthetic oil can legally include both:

Group III hydrocracked mineral oils
Group IV PAO synthetic oils

This definition stems from a 1999 ruling involving Mobil and Castrol, where the National Advertising Division (NAD) ruled that Group III oils could be marketed as synthetic due to their similar performance characteristics.

The Technical Reality

Group III oils are derived from crude oil, albeit highly processed
Group IV oils are man-made molecules with engineered uniformity

While Group III oils offer excellent performance and are found in many reputable products, PAO-based Group IV oils generally offer superior oxidative stability, volatility, and extreme temperature performance.

Why It Matters

The distinction is important for:

High-performance engines
Extended oil change intervals
Extreme operating environments

Consumers who demand the best may prefer oils that use Group IV base stocks or blended formulations with Group V esters for added performance.

Performance Differences by Group

Understanding base oil groups helps predict how a lubricant will behave under stress, heat, and over time.

Feature	Group I	Group II	Group III	Group IV (PAO)	Group V (Esters, etc.)
Refining Level	Low	Moderate	High	Synthetic	Synthetic
Viscosity Index	80–119	80–119	120+	125+	Variable
Oxidation Stability	Poor	Fair	Good	Excellent	Excellent
Cold Temp Flow	Poor	Fair	Good	Excellent	Excellent
Cost	Low	Moderate	Moderate–High	High	Very High
Label as “Synthetic”?	❌	❌	✅ (U.S./CA)	✅	✅

🔍 Note: Additive packages also significantly influence oil performance — even high-quality base oils can underperform without proper additives

Frequently Asked Questions

FAQ

What is the difference between Group II and Group III base oils?

Group III oils are more highly refined, with better cold-flow and oxidative stability. They also have a higher viscosity index (120+), making them eligible for synthetic labeling in some markets.

Are Group IV base oils always better than Group III?

Generally yes, especially under extreme conditions. However, high-quality Group III oils with strong additive packages can match or exceed some Group IV oils in consumer applications.

What are Group V base oils used for?

Group V base oils, like esters or PAGs, are used to improve performance traits such as seal compatibility, detergent action, or high-temperature resistance. They’re rarely used alone and are typically blended into high-end formulas.

Is synthetic oil always made from Group IV base oil?

Not necessarily. Many mainstream “synthetic” oils use Group III base oil due to cost and availability. True synthetic oils (based on PAO or esters) are more common in premium or racing formulations.

Why does viscosity index matter?

A higher viscosity index means the oil maintains consistent thickness across temperature changes. Oils with high VI flow better in cold weather and maintain protection in high heat.