Hydraulic Oils / 5 min
What Is ISO VG? Choosing the Right Hydraulic Oil Viscosity Class
In industrial hydraulic systems, ISO VG (International Organization for Standardization Viscosity Grade) classes, defined under ISO 3448, express an oil's resistance to flow. The right viscosity maximizes pump efficiency while minimizing wear.
Published by: Rotek Technical Content Team
Content is published based on manufacturer technical documentation and equipment requirements as the basis for final selection. This is general guidance and does not replace equipment-specific manufacturer approval.
Review our editorial guidelinesTable of Contents
- - What Is ISO VG (Viscosity Grade)?
- - Why Viscosity Matters in Hydraulic Systems
- - The Effect of Ambient and Operating Temperature
- - Choosing by Pump Type
- - Consequences of Using the Wrong Viscosity
What Is ISO VG (Viscosity Grade)?
ISO 3448 classifies industrial oils by their kinematic viscosity at 40°C (in cSt or mm²/s). For example, ISO VG 46 indicates the oil has an average viscosity of 46 cSt at 40°C. The tolerance range set by the standard is ±10% for each class.
Why Viscosity Matters in Hydraulic Systems
Hydraulic oils don't just transmit power — they also lubricate components and carry away heat. The ideal viscosity minimizes pump losses while maintaining sufficient oil film thickness between moving parts.
The Effect of Ambient and Operating Temperature
Viscosity is inversely related to temperature. In low ambient temperatures, hydraulic oil thickens (viscosity rises); at high temperatures, it thins (viscosity drops). ISO VG 32 can be preferred in cold environments for easier start-up, ISO VG 46 for general industrial settings, and ISO VG 68 for heavy-duty systems operating at higher temperatures.
Choosing by Pump Type
Piston, vane and gear hydraulic pumps have different viscosity requirements. Piston pumps may need slightly higher viscosity to prevent leakage, while vane pumps may have different tolerances. Always reference the equipment manufacturer's (OEM) technical manual.
Consequences of Using the Wrong Viscosity
Using an oil that's too thin (low viscosity) causes internal leakage, pressure loss and metal-to-metal contact due to an insufficient oil film. Using an oil that's too thick (high viscosity) increases flow resistance, raises cavitation risk, slows the system and causes overheating.
Frequently Asked Questions
At what temperatures is ISO VG 46 hydraulic oil used?
ISO VG 46 is generally used in temperate climates and enclosed hydraulic systems with normal industrial operating temperatures. Extreme cold or extreme heat may require a different class.
What's the difference between ISO VG 32, 46 and 68?
The main difference is fluidity — at 40°C, ISO VG 32 is thinner (more fluid), 46 is medium, and 68 is thicker. As operating temperature or pump pressure rises, a thicker oil (e.g. 68) is preferred.
What happens if hydraulic oil viscosity changes?
If viscosity drops (the oil thins), leaks increase and the system loses pressure; if it thickens, the pump struggles to draw the oil in and cavitation damage risk appears. Both cases increase wear and reduce efficiency.
How do I find the correct hydraulic oil viscosity?
The correct viscosity is first found by checking the hydraulic pump or system manufacturer's (OEM) maintenance manual recommendations. The second step is evaluating the system's average operating temperature.
Does a hydraulic oil's ISO class number indicate quality?
No, the ISO VG number only indicates the oil's thickness (viscosity) and has no direct link to quality. Quality depends on the base oil's properties and the anti-wear, anti-oxidation additives it contains (e.g. HLP, HVLP classes).