Part 1: Oil, the lifeblood of a hydraulic system


Functions of hydraulic oil:

The major function of hydraulic oil is to provide energy transmission through the system. However, hydraulic oil is also responsible for lubrication, cooling, sealing and removing contamination and wear materials. All of these functions need to be considered when choosing an oil.

Types of hydraulic oil:

The three most common varieties of hydraulic oils found on the market today are petroleum-based (mineral) oil, water-based oil and synthetic oil. Biodegradable oils are becoming increasingly important.

Petroleum-based oils are made by refining crude oil to a desired level. Additives are added to improve lubricant performance. These oils offer a lower cost alternative to other oil types and performance is often adequate.

Water-based oils are the least common of the fluid types. These fluids are typically required where there is a high probability of fire. They are more expensive than petroleum but less expensive than synthetics. While they offer good fire protection, they do lack wear-protection abilities.

Synthetic oil is a man-made chain of molecules that are precisely arranged to provide excellent fluid stability, lubrication and other performance-enhancing characteristics. These oils are great choices in challenging conditions (such as very high or low temperature), however they can have disadvantages such as high cost, toxicity and potential incompatibility with certain seal materials.

Demand for equipment operators to use biodegradable oils has increased recently with concern for the environment. However, these oils are still costly and suffer from technical issues such as difficulty in avoiding contamination with mineral oils as well as compatibility with seal material.

Viscosity and Viscosity index

Viscosity and viscosity index are altogether different kind of ratings and should not be confused. Viscosity is a measure of the oil’s thickness or its resistance to flow and remains the single most important characteristic of hydraulic oil. The viscosity index number is an indication of the change of viscosity with temperature.


SAE and ISO are two different scales to measure viscosity, SAE 10W is equivalent to ISO 32, SAE 20 is equivalent to ISO 46 and 68, and SAE 30 is equivalent to ISO 100.

Viscosity index

The viscosity index (VI) is denoted by a unitless number, the higher the number, the more stable the oil or the less the viscosity changes with temperature. Typical mineral based hydraulic oils have a VI range of between 90 and 105, whereas a synthetic hydraulic oil could have a VI or 160. After the viscosity, the VI is arguably the most important consideration of a hydraulic oil.


Additives are mixed in to hydraulic oil to improve their performance. Typical performance improvements are: anti-wear, anti-foaming, anti-oxidant, anti-rust and water emulsifiers. Oil additives are depleted over the life of the oil and although these additives can be replaced, it is often not advised. Good oil analysis tests can show the level of additives in the oil.

Good practice

The manufacturers of hydraulic equipment normally specify the oil requirements of their equipment. It is always good practice to follow their recommendations.

Reference on our Website

This information as well as the other parts of this series will be available on our website for  quick and easy reference.



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