Hydraulic Oil Selection and Viscosity (ISO VG) Guide

Oil is the heart of a hydraulic system; it transmits power, lubricates, cools and protects. The wrong oil returns as pump wear, rising temperature, lost efficiency and failure. The right choice starts with viscosity.

What is ISO VG?

ISO VG (Viscosity Grade) is the oil's kinematic viscosity at 40 °C. For example ISO VG 46 has ~46 cSt at 40 °C. The most common grades in industrial hydraulics are VG 32, 46 and 68.

Which VG to choose?

The choice depends on three variables:

• —Operating temperature: Hot environment/high load → higher VG (68). Cold environment → lower VG (32).
• —Pump type: Vane and gear pumps usually VG 46; high-pressure piston pumps use the maker's specified range.
• —Manufacturer's recommendation: The VG range in the machine manual is decisive; it is not changed arbitrarily.

Why does the viscosity index (VI) matter?

VI shows how viscosity changes with temperature. A high-VI oil does not thicken too much in the cold or thin too much when hot — it keeps a stable film over a wide temperature range. High VI is preferred in systems running at variable temperatures.

Additives and cleanliness

Modern hydraulic oils contain anti-wear (AW), antioxidant, rust and anti-foam additives. System cleanliness (ISO 4406 particle class) is as important as the oil itself; dirty oil quickly destroys valves and pumps. Filtration and moisture control must not be neglected.

Is a fire-resistant fluid needed?

In foundries, presses and steelworks with fire/spark risk, a fire-resistant hydraulic fluid (HFC/HFD) may be required instead of standard mineral hydraulic oil. That is a separate assessment.

Summary

First check the machine maker's VG range, narrow it by operating temperature and pump type, and complete it with high VI and a clean system. Ahmet Ceviz recommends the hydraulic oil that fits your system; use the product finder or request a sample.