Choosing the Right ISO VG Viscosity Grade
Viscosity is the single most important property of a hydraulic oil: it decides the lubricating film, efficiency and wear. Too thin and the film breaks down and the pump wears; too thick and losses rise and cold-start cavitation threatens. This guide shows how to pick the right ISO VG grade from operating temperature and pump type.
View hydraulic oilsWhat does the ISO VG grade mean?
The ISO viscosity grade (ISO VG) to ISO 3448 states the kinematic viscosity of an oil at 40 °C in mm²/s (cSt). So ISO VG 46 means around 46 mm²/s at 40 °C. The higher the number, the thicker the oil flows. The grade is the starting point of every selection because pump and valve makers tie their approvals to these values.
How do I pick viscosity by temperature and pump?
The decisive factors are the oil operating temperature and the pump type. Every maker gives an allowed viscosity range (for example 10 to 1000 mm²/s) and an optimum viscosity (often 16 to 36 mm²/s) for best efficiency. The goal is that viscosity at operating temperature sits in the optimum window and does not exceed the upper limit at cold start.
- Measure the oil operating temperature, do not assume ambient temperature.
- Base the choice on the optimum viscosity from the pump datasheet.
- Check cold start: keep start-up viscosity below the pump upper limit.
- When in doubt, pick a grade that lands in the optimum window at operating temperature.
Why do viscosity index and cleanliness matter?
The viscosity index (VI) describes how much viscosity changes with temperature. A high VI (≥ 100) keeps the oil stable across a wide range and is ideal for varying temperatures. Multigrade oils with VI above 140 allow cold start and full load on one fill. Alongside viscosity, oil cleanliness decides the service life of the components.
A high VI keeps viscosity stable over temperature, key for outdoor use.
More on fluidsMind the cleanliness class to ISO 4406, filtration protects pump and valves.
Understand cleanlinessFrequently asked questions
Which ISO VG grade is the standard in industrial hydraulics?
ISO VG 46 is the most widely used universal grade for stationary industrial systems at moderate temperatures. Cold environments often use VG 32, while high load or heat favours VG 68. The actual oil operating temperature remains decisive.
What happens if viscosity is too low or too high?
Too low viscosity lets the lubricating film break down and raises leakage and wear. Too high viscosity increases friction losses, cuts efficiency and promotes cavitation at cold start. Either extreme shortens pump life.
What is the viscosity index for?
The viscosity index shows how stable viscosity stays across temperature. A high value means little change between cold and warm. For systems with strongly varying temperature, a high-VI oil is the safe choice.
Determine the right hydraulic oil
Our hydraulics specialists help with ISO VG grade, viscosity index and pump approval for your system.
Chosen by standard
Recommendations to ISO 3448 and maker approvals.
Temperature-matched
Viscosity matched to the real operating temperature.
Pump protection
The optimum viscosity window guards against wear.
Expert advice
Hydraulics specialists support your selection.