Hydraulic oil ISO 32 vs 46 vs 68
The number is kinematic viscosity at 40 °C (ISO 3448), not SAE engine-oil grade. ISO VG 46 is the industrial default. 32 is for colder starts or high-speed vane/piston pumps. 68 is for hotter, slower, leakier systems. The OEM pump chart still wins.
ISO VG 32, 46 and 68
| ISO VG | KV at 40 °C | Typical plant use |
|---|---|---|
| 32 | 28.8–35.2 mm²/s | Machine tools, colder shops, high-rpm vane/piston pumps |
| 46 | 41.4–50.6 mm²/s | Most industrial and mobile hydraulics in temperate/hot climates |
| 68 | 61.2–74.8 mm²/s | Hot presses, older leaky cylinders, slow heavy systems |
Those bands are ISO 3448. Antiwear hydraulic oils are further classed ISO 11158 HM or DIN 51524-2 HLP. A drum labelled AW 46 is claiming both the viscosity and an antiwear additive system.
Match the pump, then the climate
Start with the pump OEM
Vane, piston and gear pumps each publish a min/max viscosity at start-up and at running temperature. That chart beats a general “use 46” rule.
Default 46 in India / GCC heat
Many systems that run VG 32 in a European shop need VG 46 or 68 once oil temperature sits at 60–70 °C all afternoon.
High-VI / multigrade hydraulic
If the same machine must start cold and run hot, a high-VI HV / HVLP fluid spans a wider window than a monograde 46.
Foam or milky oil
Wrong viscosity is one cause; water and air are others. See foaming and milky hydraulic oil.
ISO 32 / 46 / 68 FAQ
Can I mix ISO 32 and ISO 46?
You will land somewhere in between, which may still be in the pump window — or not. Topping a 46 system with 32 is a common way to lose film in summer. Drain and fill if the grade must change.
Is HLP 46 the same as AW 46?
Close enough in the field: both mean antiwear hydraulic oil at ISO VG 46. Always check the OEM list (Denison, Eaton, Bosch Rexroth) if the pump is particular.
Can I use 15W-40 engine oil in a hydraulic system?
No. Engine oils are detergent, different VI, and often foam or varnish in a hydraulic circuit. Use a hydraulic specification fluid.
Lubechem formulates AW 32 / 46 / 68 hydraulic oils for manufacturers
If you want to see how an HM/HLP fluid is built — base oil, ZDDP or ashless AW, demulsibility — start with the hydraulic formulation page.
See how a hydraulic oil is built →