Comprehensive Viscosity Guide
Viscosity Grade Classification Guide: ISO VG, AGMA & SAE J306
This comprehensive guide covers viscosity classification systems for industrial lubricants and gear oils, including ISO VG (ISO 3448), AGMA (AGMA 250.04) and SAE J306 automotive gear oil grades. It details their development, measurement methods, and practical selection criteria.
1. ISO Viscosity Classification (ISO VG)
The ISO VG system, defined in ISO 3448, classifies industrial lubricants by kinematic viscosity at 40 °C (ASTM D445). ISO VG grades range from 2 to 3200 mm²/s (cSt).
| ISO VG | Mid-Point Viscosity
(cSt @ 40 °C) |
Min (cSt) | Max (cSt) | AGMA Grade |
|---|---|---|---|---|
| 2 | 2.2 | 1.98 | 2.4 | — |
| 3 | 3.2 | 2.88 | 3.52 | — |
| 5 | 4.6 | 4.14 | 5.06 | — |
| 7 | 6.8 | 6.12 | 7.48 | — |
| 10 | 10 | 9 | 11 | — |
| 15 | 15 | 13.5 | 16.5 | — |
| 22 | 22 | 19.8 | 24.2 | — |
| 32 | 32 | 28.8 | 35.2 | — |
| 46 | 46 | 41.4 | 50.6 | 1 |
| 68 | 68 | 61.2 | 74.8 | 2 |
| 100 | 100 | 90 | 110 | 3 |
| 150 | 150 | 135 | 165 | 4 |
| 220 | 220 | 198 | 242 | 5 |
| 320 | 320 | 288 | 352 | 6 |
| 460 | 460 | 414 | 506 | 7 |
| 680 | 680 | 612 | 748 | 8 |
| 1000 | 1000 | 900 | 1100 | 8A |
| 1500 | 1500 | 1350 | 1650 | — |
| 2200 | 2200 | 1980 | 2420 | — |
| 3200 | 3200 | 2880 | 3520 | — |
2. AGMA Viscosity Classification
The AGMA system (AGMA 250.04) categorizes industrial gear oils with grades 1 through 8A corresponding directly to ISO VG classes. It emphasizes extreme-pressure performance important in gear applications.
| AGMA Grade | Kinematic Viscosity
(cSt @ 40 °C) |
Equivalent ISO VG |
|---|---|---|
| 1 | 41.4–50.6 | 46 |
| 2 & 2 EP | 61.2–74.8 | 68 |
| 3 & 3 EP | 90–110 | 100 |
| 4 & 4 EP | 135–165 | 150 |
| 5 & 5 EP | 198–242 | 220 |
| 6 & 6 EP | 288–352 | 320 |
| 7 Comp. & 7 EP | 414–506 | 460 |
| 8 Comp. & 8 EP | 612–748 | 680 |
| 8A Comp. & 8A EP | 900–1000 | 1000 |
3. SAE J306 Automotive Gear Oil Viscosity Grades
Automotive gear oils and manual transmission fluids are classified under SAE J306 (revised 1998). Viscosity grades indicate resistance to flow and are vital for ensuring adequate lubrication film thickness at operating temperature.
3.1 Winter Grades
- 70W: Max viscosity of 150,000 cP at –55 °C.
- 75W: Max viscosity of 150,000 cP at –40 °C.
- 80W: Max viscosity of 150,000 cP at –26 °C; kinematic viscosity at 100 °C ≤ 7.0 cSt.
- 85W: Max viscosity of 150,000 cP at –12 °C; kinematic viscosity at 100 °C ≤ 11.0 cSt.
3.2 Summer Grades
- 80: 7.0 < kinematic viscosity @ 100 °C < 11.0 cSt.
- 85: 11.0 < kinematic viscosity @ 100 °C < 13.5 cSt.
- 90: 13.5 < kinematic viscosity @ 100 °C < 24.0 cSt.
- 140: 24.0 < kinematic viscosity @ 100 °C < 41.0 cSt.
- 250: kinematic viscosity @ 100 °C ≥ 41.0 cSt.
3.3 Monograde vs. Multigrade
Monograde oils meet one temperature-specific requirement (e.g., 75W or 85). Multigrade oils combine winter and summer grades (e.g., SAE 80W-90) to perform across a wider temperature range.
3.4 Key Changes in 1998 Revision
- Introduction of SAE 80 and SAE 85 summer grades.
- Shear stability requirement: maintain viscosity grade after a 20‑hour CEC L-45-T-93 test.
- Standardized global viscosity labeling guidelines for clearer consumer selection.
4. Key Differences and Considerations
- Scope: ISO VG for industrial fluids; AGMA for gear oils requiring EP; SAE J306 for automotive gear and transmission fluids.
- Numbering: ISO uses VG numbers; AGMA uses grades mapping to ISO; SAE uses W and non-W numbers for cold/hot performance.
- Standards: ISO 3448; AGMA 250.04; SAE J306.
- Applications: Select based on equipment type—industrial machinery, heavy gears, or automotive transmissions.
5. Practical Selection Guidelines
- Identify operating temperature range and viscosity needs.
- For industrial/gearbox oils, choose ISO VG grade for film thickness.
- For heavy-duty or EP gears, specify AGMA grade.
- For automotive gear/transmission fluids, select SAE J306 grade per OEM recommendations.