Viscosity Index and Temperature Behavior
The viscosity of a hydraulic oil changes with temperature: thick when cold, thin when hot. The viscosity index (VI) describes how strong this change is. Matching VI, ISO VG grade and operating window correctly prevents cold-start trouble, leakage losses and efficiency drops. This guide walks through the relationships step by step.
View hydraulics basicsWhat do viscosity and viscosity index mean?
Kinematic viscosity in mm²/s (cSt) describes how thick the oil is and is stated per ISO 3448 at 40 °C as the ISO VG grade. A VG 46 therefore has about 46 mm²/s at 40 °C. The viscosity index is a dimensionless figure: the higher the VI, the flatter the viscosity-temperature curve and the more stable the oil stays across the temperature range.
How does temperature affect the oil?
As temperature drops, viscosity rises sharply: the oil becomes thick, the pump draws poorly and starved lubrication threatens at cold start. As temperature rises, viscosity falls: the lubricating film thins, internal leakage grows and volumetric efficiency drops. Every pump has a window of minimum and maximum allowable operating viscosity that must be respected.
- Optimal operating viscosity of many pumps is 16 to 36 mm²/s.
- Respect the cold-start limit and continuous-duty limit from the pump datasheet.
- Keep oil temperature between 40 and 60 °C where possible.
Which VG grade and VI should I choose?
The VG grade follows the operating temperature and pump type. Internal-gear and vane pumps often run fine on VG 46, while heavily loaded axial-piston pumps prefer VG 46 to 68. With widely swinging ambient temperatures or outdoor units, a multigrade oil with a high VI helps, because it stays pumpable when cold and load-bearing when hot.
Set the coldest start temperature and the hottest continuous temperature.
View basicsFrequently asked questions
What is the difference between ISO VG and viscosity index?
The ISO VG grade states the viscosity at 40 °C, that is how thick the oil is at a fixed temperature. The viscosity index describes how strongly that viscosity changes with temperature. A high VI means a flat, stable curve.
What viscosity should my hydraulic oil have in service?
Many pumps work best between 16 and 36 mm²/s. Viscosity too high at cold start causes cavitation, too low at overheating causes leakage and wear. Keep the oil temperature between 40 and 60 °C where possible.
When is a high-VI multigrade oil worthwhile?
With widely swinging temperatures, cold starts or outdoor units. A high VI keeps the oil pumpable when cold and load-bearing when hot, covering a wider operating window without a seasonal oil change.
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