Base oil groups explained (Groups I–V)
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API classifies base stocks into Groups I–V using defined properties or chemical families. A group number does not establish a manufacturing route, a finished oil’s performance, or vehicle compatibility. Compare the finished oil’s SAE grade and required specifications with the manual.
What API classifies
API 1509, Annex E.1.2–E.1.3, distinguishes a base stock from the base oil or blend used in a finished lubricant. The limits below classify base stocks; VI means viscosity index.
| Group | Classification criteria |
|---|---|
| I | Saturates <90% OR sulfur >0.03%; 80 ≤ VI <120 |
| II | Saturates ≥90% AND sulfur ≤0.03%; 80 ≤ VI <120 |
| III | Saturates ≥90% AND sulfur ≤0.03%; VI ≥120 |
| IV | Polyalphaolefins (PAO) |
| V | Base stocks outside Groups I–IV |
API 1509 · 24th ed. · E.1.2–E.1.3 · pp. 57–58.
What the groups do not establish
API permits different manufacturing processes. Do not infer a fixed refining method, feedstock, formulation percentage or wear result from a group number alone. Group V is the remainder of the classification, not a higher performance tier.
Reading a synthetic label
The word “synthetic” alone does not identify the exact base-stock mixture. This reference does not establish how a named product was made or whether a particular marketing claim is valid in a given market; that needs product-specific evidence.
Choosing the finished oil
Record the SAE grade, performance category and any OEM approval from the manual. Check those fields on the finished product. A base-stock group cannot supply a missing approval or authorize a longer change interval.
Frequently asked questions
Does Group III specify a manufacturing process?
No. The API definition uses saturates, sulfur and viscosity index. A product’s production route needs separate evidence.
Is a higher group always better?
No. Group numbers classify base stocks; they are not a ranking of finished-oil protection.
Can the group identify the right oil for my car?
No. Use the finished oil’s allowed SAE grade and required performance specifications.