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What to consider when choosing Hydraulic Oil

Hydraulic oil is a liquid used to transfer power in hydraulic machinery. It is formulated to endure the constant pressure it is put through, ensuring optimal performance. On top of that, it provides a lot of lubrication to the system, which helps reduce wear and tear from friction.

As such, hydraulic oil is irreplaceable in a hydraulic system and simply will not work without it. However, not all hydraulic oil is equal. There are various factors to consider when choosing the oil that you need.



Viscosity means thickness. High viscosity means hydraulic oil that is thick, sticky, harder to move etc., whereas low is the opposite. The state of hydraulic oil’s viscosity is a crucial thing to understand, as it dictates how much lubrication and power transmission ability it has. You need to have the right level in your oil, to ensure that the machine is operating smoothly. Too much viscosity impedes the machine’s flow of power, whereas too low means metal-to-metal contact is more common due to the lack of lubrication, and wear and tear will be increased by a large magnitude.

Temperature impacts viscosity quite a bit. As temperatures rise, the oil becomes less viscous, meaning it gets thinner. This means that the overall working temperature of your hydraulic vehicle will have a big impact on your oil’s viscosity.

Manufacturers of hydraulic oil grade their oil’s viscosity based on the International Standards Organisation’s (ISO) ISO Viscosity Grades (ISO VG). Each of these grades rate their oil’s viscosity when at a heat of 40 Celsius. There are quite a few grades, and to make things easy, hydraulic machines often come with their own recommendation, making the selection process a lot easier.



Additives are essentially multi-vitamins for the hydraulic oil – they add to the properties that the oil both already has and doesn’t.

Unlike in food, additives don’t have drawbacks within the oil. They more or less universally improve most aspects of it. You will ensure that with these additives, you will have a generally stronger system, and have oil that retains its full potential for much longer before degrading.

Common Additives

The following additives will give you an idea of the types of benefits you can expect. 


  • Anti-Wear Agents – Forming a protective layer on the surfaces of metal, they ensure that these layers are what come into contact with each other, rather than the metal surfaces themselves.
  • Rust and Corrosion Inhibitors – Fairly self-explanatory, they coat the surfaces to ensure that rust and corrosion have a harder time taking root in the system.
  • Oxidation Inhibitors – The oxidation of hydraulic oil can lead to increased viscosity, causing sludge and a reduced transfer of power. By inhibiting oxidation, these additives keep the oil a lot more stable.
  • Foam Reducers – Foam in a hydraulic system can cause a reduction in efficiency, to the point that a system may even be damaged. This is why this additive is almost always in oil.


Most Useful Additives per System


Mobile Hydraulics

  • Anti-wear agents are useful due to the frequent start/stop nature of hydraulic machinery.
  • Rust and corrosion inhibitors due to exposure to the elements.
  • Foam reducers as rapid movements of parts in mobile hydraulics can cause foam.

Industrial Hydraulics

  • Oxidation inhibitors are useful in machines with prolonged operation times.
  • Anti-wear agents are also useful in machines with prolonged operation times.

Marine Hydraulics

  • Corrosion inhibitors are all but required in a marine setting due to the high amount of saltwater, which speeds up corrosion.
  • Demulsifiers are included to separate water ingress quickly.

Food and Beverage Industry Hydraulics

  • Food-grade additives are used to ensure that food is not ruined or contaminated in case of accidental contact.
  • Additives such as anti-wear and oxidation inhibitors are used due to prolonged use, but they are often specialised to meet food safety standards.

Aerospace Hydraulics

  • Additives that keep the system and fluid stable during the high-temperature conditions brought about mid-flight.
  • Foam reducers are all but required, as foam can interfere with the precision aerospace hydraulic systems require.

Mining Equipment Hydraulics

  • Additives that offer the highest anti-wear properties in the market are used, due to the heavy-duty nature of mining.
  • Additives that control dust and condition seals may be added due to the high amounts of dust in mining operations.


Base Oils

There are three types of base oil. 


  • Mineral Oils – These come from crude oil refineries and are considered the most traditional type of oil. They are widely available, which also means they are the most cost-effective option and suitable for a lot of different systems. That being said, they have a limited performance range and are more susceptible to oxidation. Whilst this is not the case for all of them, some have impurities.
  • Synthetic Oils – Man-made oils that are made to meet specific desired properties, and are usually far less volatile as a result. They have superior thermal stability and flow better at lower temperatures. Synthetic oils are more expensive than mineral oils and may suffer from compatibility issues with certain seals or older equipment.
  • Bio-based Oils – These are made from renewable resources, like plants or animal fats, and can be easily bio-degraded. Their environmentally good nature is not the only reason to acquire some, as they have good natural lubrication. Unfortunately, their narrow temperature operating range may disqualify their use by a lot of machines. They can also be more expensive or may require more frequent changes due to weaker oxidative stability.


Which do I need?

Consider the following before resigning yourself to any one option: 


  • Temperature Extremes – Environments that have high temperatures as a standard (as in within a hot country during summer) may require oils with higher temperature stability, such as synthetic oils.
  • Eco Sensitivity – Areas in which the environment is protected means that you will require bio-based oils, to ensure that the oil waste is biodegradable and won’t clog up the area.
  • General Industry Use – If you are using hydraulic oil in a standard industrial setting, without any extreme factors such as temperature, mineral oils will often suffice and be more cost-effective.
  • High-Performance Needs – In situations that demand that you bring out the best oil you have to ensure high efficiency, you would be better off with synthetic oils.


Thermal Stability 

Thermal stability refers to the ability of oil to stay stable at varying levels of temperature. Elevated temperatures in inferior oils tend to cause breakdown in the oil itself, which also means lubrication dies, increasing wear and tear. Broken down oil turns to sludge, varnish and other types of harmful deposits that only damage the system’s performance.

Oils that have good thermal stability tend to last much longer, as having high thermal stability means having good resistance against oxidation, and the degradation that it brings. If this is what you need, look toward the following oils:


  • Synthetic Base Oil – Synthetic oils have high thermal stability compared to mineral oils.
  • Oils with Oxidation Inhibitors – Oils that include additives that inhibit oxidation within the system will stop oxidation from degrading the oil.
  • Higher Viscosity Oils – High viscosity means there will be less breakdown at high temperatures compared to low.


Water Contamination and Demulsibility 

Water in your hydraulic system has many disastrous results.


  • Corrosion – Water against metal promotes rusting of the internal components within the system, which is bad for obvious reasons.
  • Reduced Lubrication – Water thins oil, which in turn means that the ability to apply a layer of lubrication to the metal components is heavily reduced.
  • Cavitation – In zones of low pressure, water can create vapour cavities. When they collapse, they often cause damage to the machine.
  • Hydraulic Component Malfunction – Precise operation of hydraulic components is very important, and water can interfere with this.
  • Accelerated Degradation – Water can push oxidation, which is considered the devil to hydraulic systems.


The Importance of Demulsibility

Demulsibility refers to the ability of oil to separate from water. Sometimes, water makes its way into engines, usually not in large amounts. But what amount there is can cause trouble – which is why it’s very important for oil to have good demulsibility. If you can get oil that has good demulsibility, you can ensure any water that does get into your hydraulic system can easily be separated. This will prevent negative effects on your system, and ensure that it maintains optimal operational efficiency.

This is particularly important in areas that have high water content. Considerations of which oil to buy include:


  • Demulsifier Additives – Oils that are fortified with additives that promote demulsification are a good option.
  • Synthetic Oils – Some synthetic oils, by the very makeup of the material, possess massive levels of demulsification.
  • Regular Monitoring – Operations in environments that are prone to moisture should include regular checks of oil content.
  • Seal Quality – Seal failure is the most common source of water ingress, so investing in good seals is a fantastic way of circumventing the issue.
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