Base Oil Groups I–V
Every lubricant starts with a base oil — 70–99% of the finished product. The American Petroleum Institute (API 1509) sorts base stocks into five groups by how refined they are, measured on three numbers: saturates, sulfur and viscosity index. The group you choose sets the ceiling on oxidation life, cold flow, volatility and cost before a single additive goes in. Here is what each group is, and how to pick one.
Three numbers define a base stock
API classifies base oils on three measurable properties. Together they predict how a base stock behaves before additives are added:
- Saturates (ASTM D2007) — the fraction of saturated hydrocarbons. More saturates means better oxidation and thermal stability.
- Sulfur (ASTM D2622/D4294) — residual sulfur from crude. Lower sulfur generally means cleaner, more stable oil (and matters for after-treatment compatibility).
- Viscosity Index, VI (ASTM D2270) — how little the viscosity changes with temperature. A higher VI holds grade across cold starts and hot operation with less VI improver.
The base oil is the single biggest lever in a formula. Our formulation guide shows where it sits in the recipe; this page is the base-stock detail behind it.
API 1509 base oil groups at a glance
| Group | Saturates | Sulfur | VI | Made by / typical |
|---|---|---|---|---|
| Group I | < 90% | > 0.03% | 80–120 | Solvent-refined mineral |
| Group II | ≥ 90% | ≤ 0.03% | 80–120 | Hydrocracked mineral |
| Group III | ≥ 90% | ≤ 0.03% | ≥ 120 | Severe hydrocracking / GTL |
| Group IV | Polyalphaolefin (PAO) — a true chemically-synthesised base | Synthetic (PAO) | ||
| Group V | All base stocks not in I–IV — esters, PAG, naphthenic, white/silicone oils | Speciality synthetics | ||
Informal “Group II+” and “III+” labels mark the higher-VI end of each group (roughly VI 113–119 and 130+). They are marketing shorthand, not separate API groups.
What each group is really for
Group I — the economy base
Solvent-refined. Highest additive solubility and lowest cost, but weaker oxidation stability and higher volatility. Still ideal for many industrial oils, greases and process oils where extreme temperatures are not in play.
Group II — the modern workhorse
Hydrocracked to remove aromatics and sulfur. Clean, stable and the default for most current engine and industrial oils — a large stability gain over Group I at modest cost.
Group III — “synthetic” mineral
Severely hydrocracked or GTL, VI 120+. Marketed as synthetic in many markets. The base of most full-synthetic passenger-car engine oils — excellent VI and volatility at a fraction of PAO cost.
Group IV — PAO
True synthetic polyalphaolefin. Very high VI, very low pour point and low volatility — for wide-temperature, long-drain and low-temperature duty. More expensive; often blended with esters to restore additive solubility.
Group V — the speciality shelf
Everything else: esters (solubility, biodegradability), PAG (high VI, water-tolerant), naphthenics (solvency, low pour) and white/USP oils. Usually a co-base, not the whole base.
How to choose the base oil for a formula
Base-stock choice is a balance of five demands against cost:
- Temperature window — cold-start pour point and high-temperature oxidation set the minimum group. Wide windows push toward III/IV.
- Volatility (Noack, ASTM D5800) — lower Noack means less oil consumption and cleaner engines; Group III/IV win here.
- Additive & seal compatibility — Group I dissolves additives best; PAO can shrink seals and dissolve additives poorly, so a few percent ester is added.
- Regulatory / claim — API/ACEA and OEM specs, biodegradability (EAL) or food-grade (NSF H1) narrow the choice.
- Cost per litre of performance — the goal is the lowest-cost base that clears the spec, often a blend of two groups.
Base-stock selection with Lubechem
We pick and blend base stocks to hit your viscosity grade, VI, volatility and cost target — then build the matching additive package and prove it in the lab. Whether you are launching an engine oil, an industrial fluid or a bio-lubricant, the base-oil decision is where we start. You own the formula and the IP.
Related guides
Base oil groups FAQ
Is Group III a synthetic oil?
Legally, in many markets, yes — Group III can be marketed as “synthetic” even though it is made from crude by severe hydrocracking. Only Group IV (PAO) and most Group V stocks are chemically synthesised. Group III delivers near-PAO viscosity index and volatility at much lower cost, which is why most full-synthetic engine oils today are Group III based.
What is the difference between Group I and Group II?
Group I is solvent-refined and keeps more aromatics and sulfur (saturates below 90%, sulfur above 0.03%). Group II is hydrocracked to at least 90% saturates and 0.03% sulfur or less. Group II is cleaner and far more oxidation-stable, while Group I has better additive solubility and lower cost.
Why add ester to a PAO (Group IV) oil?
PAO has poor additive solubility and can shrink elastomer seals. Blending 5–15% of a Group V ester restores additive solubility, conditions seals and lifts polarity — giving the temperature performance of PAO without the solubility drawbacks.
Which base oil group has the highest viscosity index?
Group III and Group IV (PAO) have the highest VI, typically 120 and above, with some Group V stocks (like PAG) higher still. Higher VI means the oil holds its grade across a wider temperature range with less VI-improver additive.
Which group is best for a lubricant?
There is no single best group — it depends on the temperature window, volatility target, seal and additive compatibility, any regulatory claim, and cost. Most finished oils use Group II or III, often blended with a little Group V ester; only demanding low-temperature or long-drain oils justify Group IV PAO.
Choose the right base stock before you formulate.
From Group I economy oils to PAO/ester blends — we select and blend the base oil to your grade, VI and cost target, then build and test the full formula. The IP is yours.
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