API Group I base oil refining begins by vacuum distillation of the feedstock to separate heavy hydrocarbons from lighter fractions. The heavy cut is then treated with solvent extraction to remove aromatic compounds and undesirable impurities. Next it undergoes solvent dewaxing to eliminate long‑chain wax molecules that impair cold‑temperature performance. Following dewaxing, the oil is hydrofinished in a hydrogen‑enriched, high‑temperature/high‑pressure environment to saturate unsaturated structures and eliminate free radicals. The result is a higher‑purity hydrocarbon base oil of improved oxidative stability. This conventional solvent‑refining pathway defines typical Group I stocks such as solvent neutrals and bright stock. In some plants, an alternative refinement route employs hydrotreating and catalytic dewaxing—resembling Group II techniques—especially when processing naphthenic crude. But the classic Group I method relies on vacuum distillation, solvent extraction, solvent dewaxing and hydrofinishing to deliver a stable paraffinic base oil suitable for industrial applications.
Group II base oil production starts with hydroprocessing to replace solvent extraction. Heavy vacuum‑distillate fractions are routed into a hydrocracker where hydrogen under high pressure and temperature cracks aromatics and waxy molecules into more stable paraffinic and naphthenic hydrocarbons. Next the oil is dewaxed not by solvent chilling but via catalytic dewaxing or wax isomerization. Catalytic dewaxing breaks wax into gases or naphtha reducing yield; isomerization converts wax into branched paraffins raising yield and improving low‑temperature flow. Finally the oil is hydrofinished to saturate remaining unsaturated molecules and remove reactive compounds. The result is a base oil with more than 90 percent saturates less than 0.03 percent sulfur and a viscosity index between 80 and 120. The process delivers a more uniform stable product with better oxidative and thermal stability than Group I. It also offers flexibility in feedstock selection though it requires substantial capital investment for hydrocrackers.
Group III base oil is produced via severe hydrocracking, catalytic dewaxing and hydrofinishing applied to high‑quality vacuum‑distillate feedstocks. Hydrocracking under elevated temperature and pressure breaks aromatic and wax molecules into highly saturated paraffinic structures raising viscosity index above 120. The catalytic dewaxing step converts long‑chain wax into iso‑paraffins instead of discarding wax, improving low‑temperature flow while preserving yield. Final hydrofinishing removes residual unsaturates and heteroatoms saturating molecules and enhancing oxidative stability. The resulting base oil contains at least 90 percent saturates no more than 0.03 percent sulfur and achieves a water‑white clarity. These processes elevate Group III to synthetic‑like performance, matching or exceeding polyalphaolefins in many applications while remaining classified as a mineral base stock per API standards.
Naphthenic base oil refining begins with vacuum distillation of atmospheric residue to isolate heavier fractions designated for lubricant cuts. Because naphthenic crudes contain few linear paraffins, dewaxing is typically unnecessary. The distillate then undergoes solvent extraction or hydrotreating to remove aromatic compounds sulfur and nitrogen compounds and improve stability. Final hydrofinishing saturates unsaturated molecules enhancing oxidative resistance and color clarity. Naphthenic base oils yield intermediate viscosity index low pour points (often below –70° C) and high solvency making them ideal for process oils transformer oils metalworking fluids and greases. Their strong solvency supports additives resins and contaminant dissolution without extra solvent boosting. Because they require fewer refining steps than paraffinic oils they can be cost‑effective where low VI is acceptable and solvency or low‑temperature flow is valued.
API Definitions
| API GROUP | Sulfur % Wt. | Saturates % | Viscosity Index | typical applications | |
|---|---|---|---|---|---|
| Group I | > 0.03 | And/or | < 90 | 80-119 | Marine, industrial, process oils, older passenger car motor oils |
| Group II | ≤ 0.03 | And | ≥ 90 | 80-119 | Industrial, 10W-XX passenger car motor oils, 15W-XX heavy-duty diesel engine oils |
| Group III | ≤ 0.03 | And | ≥ 90 | ≥ 120 | 0W-XX and 5W-XX PCMO and HDMO |
| Group IV | 0.03 | All Polyalphaolefins (PAOs) | All Polyalphaolefins (PAOs) | All Polyalphaolefins (PAOs) | PCMO and industrial |
| Group V | All base stocks not in Group I-IV (naphthenics, non-PAO synthetics) | All base stocks not in Group I-IV (naphthenics, non-PAO synthetics) | All base stocks not in Group I-IV (naphthenics, non-PAO synthetics) | All base stocks not in Group I-IV (naphthenics, non-PAO synthetics) | Process oils, transformer oils, industrial lubricants, gear oils, transmission fluids, compressor oils, engine oils |
What Are Group II+ and III+?
| Industry-established “categories” originally developed to describe base oil suitable for SAE 10W-XX and 5W-XX multigrade motor oil blending. They are marketing terms, not official API definitions. |
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| • Group II+ is generally recognized as referring to Group II oils with viscosity index of 112 to 119 and Group III+ as referring to Group III oils with VI of 130 or greater. |
| • Viscosity grades are tailored to making passenger car motor oils. |
| • More recently, companies have begun marketing Group I+. The same principal of higher-end viscosity index applies. |