Group I, II, III and Re-Refined: Which Base Oil Does Your Formulation Actually Need?

Base oil selection is one of the few decisions in lubricant blending that touches everything downstream — additive response, finished-product performance, cost per litre, and which customers you can credibly serve. Yet the classification system that governs it is frequently misunderstood, and one misconception in particular costs buyers money: the belief that "re-refined" is a category alongside Group I and Group II.

It isn't. Here is how the system actually works, and where each option genuinely belongs in a blend plant's sourcing strategy.

The API classification, in plain terms

The American Petroleum Institute classifies base stocks into five groups. Three parameters do the work: saturates content, sulphur content, and viscosity index (VI).

Two things are worth noticing in that table.

First, Group I and Group II share the same VI band. The difference between them is not viscosity behaviour — it is molecular cleanliness. Group II's higher saturates and lower sulphur come from hydrotreating, which converts unstable aromatic and sulphur-bearing molecules into stable saturated ones.

Second, the classification says nothing about feedstock. It describes what the finished base oil is, not where the molecules came from. That single fact is the key to the re-refined question.

What each group is actually good for

Group I remains in service across older industrial formulations, particularly where bright stock is needed for the heaviest viscosity grades, and where the higher aromatic content gives useful additive solvency. Its weaknesses are oxidation stability and volatility relative to Group II. Global Group I capacity has been shrinking for years as refineries close or convert, and the 2026 supply disruptions made that structural tightness visible in a way the market had been able to ignore. Bright stock specifically is a co-product of Group I plant architecture rather than a grade any Group II unit can produce — see why bright stock and SN500 stay structurally short for what that means for marine, gear and grease buyers.

Group II is the workhorse of modern industrial and automotive blending. Better oxidation stability, lower volatility, near-colourless appearance, and strong response to modern additive packages make it the default for hydraulic oils, industrial gear oils, and a broad span of engine oil formulations. For most blend plants in Southeast Asia, Group II is where the majority of base oil intake sits.

Group III is where performance formulations live. A VI above 120 means the oil holds its viscosity across a wider temperature range, which is what modern low-viscosity engine oils require. Group III enables 0W-20 and 5W-30 formulations, premium synthetic and semi-synthetic products, and applications with demanding OEM specifications. It costs more, and it buys capability that Group II cannot deliver.

Groups IV and V serve specialised requirements — extreme temperature range, specific chemical compatibility, food-grade or biodegradable applications — at prices that keep them out of general industrial blending.

Where re-refined base oil actually fits

Now the correction that matters.

Re-refining is a production route, not a classification. It takes used lubricating oil as feedstock and processes it — pre-treatment and dewatering, vacuum distillation, then hydrotreating and finishing — through the same core technology used in virgin base oil plants. What comes out is determined by how deep that processing goes.

Run properly, with full multi-stage hydrotreating, the output meets Group II specifications: saturates above 90%, sulphur below 0.03%, VI in the 95–120 range, water-white in appearance. Some operations reach Group III. Run shallowly — distillation only, or clay treatment — the output does not meet Group II at all, and belongs in a different price band and a different application set.

This is why the question "is re-refined base oil Group II?" has no single answer. It depends entirely on the plant. And it is why the certificate of analysis, not the label, is the document that settles it. Our guide on how to read a re-refined base oil certificate of analysis covers the seven parameters that separate genuine Group II re-refined material from under-processed product.

The practical consequence for a buyer: re-refined Group II should be evaluated against virgin Group II on specification, not against a mental category of "recycled oil." When it meets the same parameters, it does the same job in the blend — a point covered in detail in our guide to formulating hydraulic and gear oils with re-refined base stock.

A sourcing framework

For a blend plant deciding where each group belongs:

Use Group III where the formulation demands it — low-viscosity engine oils, premium synthetics, OEM-specified products. This is a capability purchase, not a cost decision.

Use Group II as the volume base for hydraulic oils, industrial gear oils, and mainstream automotive products. This is where most of the tonnage and most of the cost sits.

Evaluate re-refined Group II inside that volume band, on specification, as a differently-sourced origin of the same material. Two things follow: a cost advantage driven by market perception rather than performance, and supply diversification — a second origin that does not share the crude-linked disruption risk of virgin supply, as buyers who lived through the 2026 Hormuz episode now appreciate. Where that supply is actually available is covered in our look at demand markets.

Treat Group I as a legacy and bright-stock requirement, sourced deliberately rather than by default, given the direction of global capacity.

The question worth asking your supplier

Whatever group you are buying, the diligence question is the same, and it is not "which group is this?" It is: "show me the batch certificate of analysis." Group classification is a claim. Saturates, sulphur, viscosity index and colour on a batch-specific document are evidence. Suppliers who lead with the second do not need to argue about the first.

Frequently Asked Questions

Sanyang Petroleum supplies Group III, virgin and re-refined base oils to lubricant blenders and industrial users across Southeast and South Asia, with batch-specific certificates of analysis as standard. To discuss grades, specifications and current availability, contact our trading desk at info@sanyangpetroleum.com.