Industrial MRO Reference Library · Lubricants & Compatibility
ISO Viscosity Grades & Oil Selection
How the ISO VG system classifies industrial oils by kinematic viscosity at 40 °C, the allowed range of each grade, and how to use the grade to match or cross-reference an oil.
Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.
What it is
ISO 3448 sorts industrial liquid lubricants — hydraulic oils, gear oils, compressor oils, way oils, circulating oils — into viscosity grades (ISO VG). Each grade number is the midpoint kinematic viscosity in centistokes (mm²/s) at 40 °C, and an oil qualifies for that grade if its measured viscosity falls within ±10% of the midpoint. ISO VG 46 means about 46 cSt at 40 °C. The system says nothing about additives, base oil type or performance — only viscosity.
How to identify
Look for "ISO VG," "ISO," or a bare grade number in the product name (for example "AW 46" or "EP 220"). The product data sheet lists measured viscosity at 40 °C and at 100 °C and the viscosity index (VI), which shows how much the viscosity changes with temperature. Automotive oils use SAE grades (SAE J300 for engine oils, SAE J306 for gear oils) instead; those are a different system and do not convert one-for-one.
Common ISO VG grades
Ranges are the ±10% band around each midpoint.
| ISO VG | Midpoint at 40 °C (cSt) | Allowed range (cSt) | Common uses |
|---|---|---|---|
| 22 | 22 | 19.8–24.2 | Light hydraulic, spindle oils |
| 32 | 32 | 28.8–35.2 | Hydraulic systems, air tools, compressors |
| 46 | 46 | 41.4–50.6 | Hydraulic systems (most common), compressors |
| 68 | 68 | 61.2–74.8 | Hydraulic systems in warm service, way oils, compressors |
| 100 | 100 | 90–110 | Compressors, circulating oils |
| 150 | 150 | 135–165 | Gear drives, circulating oils |
| 220 | 220 | 198–242 | Enclosed industrial gear drives (very common) |
| 320 | 320 | 288–352 | Gear drives under heavier load |
| 460 | 460 | 414–506 | Worm gears, slow heavy gears |
| 680 | 680 | 612–748 | Worm gears, very slow heavy gears |
Key points
- Select viscosity from the equipment maker's specification. Oil type (AW hydraulic, EP gear, compressor, food-grade) matters as much as the grade.
- When cross-referencing, match both the ISO VG and the oil type and performance specification. An ISO VG 68 gear oil is not a substitute for an ISO VG 68 hydraulic oil.
- High-VI and synthetic oils thin less when hot and thicken less when cold; they may allow wider temperature ranges in the same grade.
- Mixing base oil types can cause problems — PAG oils in particular must not be mixed with mineral or PAO oils.
- Moving one grade up or down changes viscosity by roughly half again or a third less; do not "upgrade" viscosity without approval.
Common mistakes
- Assuming an SAE gear or engine grade number equals an ISO VG number.
- Topping off hydraulic systems with any oil of the right grade but the wrong type.
- Ignoring operating temperature when the equipment maker specifies different grades for summer and winter.
- Mixing PAG-based compressor or gear oil with mineral oil.
Governing standards: ISO 3448 (industrial liquid lubricants — ISO viscosity classification). Equipment maker's lubrication specification; lubricant maker's product data sheet.
Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.