Hydraulics & Fluid Power Reference Library · Fluids, Filtration & Contamination
Hydraulic Fluid Types & ISO Viscosity Grades
The main hydraulic fluid families from ISO 6743-4 and how ISO viscosity grades (VG) are defined. It explains why the equipment maker's viscosity window, not a habit, should decide the grade.
Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.
What it is
A hydraulic fluid transmits power. It also lubricates, cools and carries contamination to the filters. Fluids are classified by type (ISO 6743-4) and by viscosity grade (ISO 3448). An ISO VG number is the midpoint kinematic viscosity at 40 °C in mm²/s (cSt), and a fluid meets that grade if it is within ±10 % of the midpoint. "AW 46" means an anti-wear hydraulic oil in ISO VG 46.
How to identify
- Product data sheet or label: gives the ISO VG, the ISO 6743-4 category (e.g. HM, HV), the OEM approvals and the viscosity index.
- Equipment manual: gives the allowed viscosity range at operating temperature and the approved fluid types. This is the controlling document.
- Reservoir tag or lube chart: many plants label the fluid at the fill point. If there is no tag, sample and test before topping up with anything.
Fluid families (ISO 6743-4)
- HH / HL: non-inhibited mineral oil / mineral oil with rust and oxidation (R&O) inhibitors.
- HM: HL plus anti-wear additives. This is the most common "AW" hydraulic oil.
- HV: HM with a high viscosity index, for wide temperature ranges (mobile and outdoor equipment).
- HFAE, HFAS, HFB, HFC, HFDR, HFDU: fire-resistant fluids (water-based, water-glycol and synthetic). Each has its own seal, pump and filter compatibility rules.
- HETG, HEES, HEPG, HEPR: environmentally acceptable fluids (ISO 15380) for sensitive sites.
ISO viscosity grades
| ISO VG | Midpoint at 40 °C (mm²/s) | Range at 40 °C (mm²/s) |
|---|---|---|
| 10 | 10 | 9.0–11.0 |
| 15 | 15 | 13.5–16.5 |
| 22 | 22 | 19.8–24.2 |
| 32 | 32 | 28.8–35.2 |
| 46 | 46 | 41.4–50.6 |
| 68 | 68 | 61.2–74.8 |
| 100 | 100 | 90.0–110 |
Key points
- Choose the grade by operating temperature. Pump makers publish minimum, optimum and maximum viscosities, plus a cold-start limit. The right grade keeps the fluid inside that window at the actual operating temperature. This is why one machine can run VG 32 in winter and VG 68 in a hot plant.
- Viscosity index (VI) shows how much viscosity changes with temperature. A higher-VI fluid changes less.
- Do not mix fluid types. Mixing mineral oil with fire-resistant or biodegradable fluids, or zinc and zinc-free AW oils, can cause sludge, seal failure or loss of fire resistance.
- Changeovers between fluid types usually need a flush and may need new seals and filters. Follow the fluid and equipment makers' procedures.
- An SAE engine oil grade (e.g. 10W) is a different scale and not interchangeable with ISO VG without data.
Common mistakes
- "It's all hydraulic oil": topping up with whatever is on the shelf.
- Picking VG 68 "for more protection" on a cold-climate machine.
- Ignoring OEM fluid approvals during warranty.
Governing standards: ISO 3448 (industrial liquid lubricants: ISO viscosity classification); ISO 6743-4 (classification, family H); ISO 11158 (mineral oil hydraulic fluid specifications); ISO 12922 (fire-resistant fluids); ISO 15380 (environmentally acceptable fluids).
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.