Hydraulic Oil – The Ultimate Guide

Hydraulic Oil

You can find hydraulic systems in very many applications ranging form heavy machinery to braking systems in vehicles. For such systems, you will require hydraulic oil to convey mechanical energy throughout your set-up.

We are going to conduct a critical analysis of hydraulic oil from its composition and properties to its applications.

What Is Hydraulic Oil

Hydraulic oil is normally a non-compressible fluid that we use in hydraulic systems to convey mechanical power and energy. They are either synthetic or mineral-based oils that you can also use either as a sealant or a lubricant.

This oil has very many ingredients that are mixed in ratios determined by the type of oil you want. The common ingredients you will find include ethers and esters, portions of mineral oil, glycol, and silicone.

Benefits of Hydraulic Oil

You can get different benefits from different types of hydraulic oil depending on how you apply them. Additionally, the type of additive incorporated in your oil also determines the specific benefits of your oil.

Let us discuss in general the benefits you can get from any type of hydraulic oil:

Hydraulic Fluid
Hydraulic Fluid
  • This oil protects your hydraulic system from any form of corrosion or rust. This is made possible by the fact they are hydrolytically stable.
  • They can work efficiently even in extreme application temperatures. Their viscosity gives them the stability to withstand such temperatures
  • It enhances the filtration efficiency of your hydraulic system. They can suspend and eliminate the contaminants in your system assisted by their filters.
  • With the inclusion of some additives, they can reduce your system’s wear and tear. This extends the service life of your equipment and system.
  • Some hydraulic oil types have unique anti-oxidation traits. This means that your hydraulic system is fully protected from exposure to water or air.
  • They protect your equipment against water emulsion damage. This is because they have a property called demulsibility that enables them to separate water and avert damage.

Properties of Hydraulic Oil

Hydraulic oil has several unique properties that enable it to efficiently operate in certain hydraulic system conditions. However, you should note that not all these properties are available in a single hydraulic oil type.

This is because the formulations for the oil are usually based on the application conditions of your hydraulic system. Let’s just have a brief overview of some of the notable properties:

Hydraulic System with Fluid
Hydraulic System with Fluid
  • Durable
  • It has anti-wearing characteristics
  • Cannot be contaminated by water
  • It is non-corrosive
  • Nn-compressible
  • Resistant to fire
  • Has a high viscous stability. This ensures that it floes smoothly within your hydraulic system while providing sufficient lubrication.
  • Can operate under a wide temperature range from low to high

Mineral-Based Vs Synthetic-Based Hydraulic Oils

We get mineral-based hydraulic oils by extracting them from crude oil. The hydraulic oil then undergoes an extensive refining process that enhances its lubrication properties.

The oil is then blended with various additives depending on where it is going to be applied. The additives enable the hydraulic oil to carry out different functions in different applications.

They can enhance the oil’s anti-wear properties, their ability to reduce rust and oxidation, and improve the oil’s viscosity index.

The fact that they are petroleum-based means that you can acquire them at a cheaper cost compared to synthetic-based ones. Their performance is usually very impressive.

As for the synthetic-based hydraulic oils, they make up for any shortcomings that their mineral-based counterpart has. These oils are manufactured using chemically produced base oils.

Compared to mineral-based oils, they perform much better and are more efficient. You can use them in applications having very high temperatures with ease.

They are also biodegradable and have good oxidation stability. The downside of it all is that you will get them at a higher cost compared to the mineral-based ones.

Another shortcoming is that it may prove to be highly toxic in some applications. It is also not compatible with several seal materials hence its application is limited to some hydraulic systems.

Types of Hydraulic Oils

Hydraulic System
Hydraulic System

We can classify hydraulic oils as either mineral-based or synthetic oils. We have already discussed the difference between the two.

We can now have a broader look at the different types of hydraulic oils that fall under the two categories.

· Fire-Resistant Hydraulic Oil

This oil type is composed of both synthetic and mineral-based hydraulic oils in its formulation. This fluid-based oil is biodegradable and can do so at a very high rate.

Just as the name suggests, we use this oil in applications that are highly prone to fire such as the aviation industry or steel milling. This trait enables it to be applied in very many industries backed up by the fact that you can get them cheaply.

· Aircraft Hydraulic Oils

As the name suggests, you will find them mostly in hydraulic systems used in the aviation industry. They are made in such a way that they can withstand extreme climatic conditions while working under very high pressure.

You will find that they have very good thermal stability paired with a high viscosity index. Their operation requires that they are free from contaminants.

· Mobile Hydraulic Oils

These are the oils that you can use in automotive machinery such as tractors and heavy vehicles. You can apply them in the gearboxes of these equipment and their hydraulic systems.

· Food-Grade Hydraulic Oils

There is an international guide on the additives and base oils that you can use when formulating these oils. We normally use them in the food industry where they can either have no direct contact with the food or small contact.

Examples of the base oils you can use for this oil include a special type of polyglycols or white oil.

· Biodegradable Hydraulic Oil

This hydraulic oil is formulated with emphasis on the impact of such oils on the environment. These can degrade rapidly therefore offering some form of environmental protection.

They are very compatible with the ecosystem with their applications varying from stationery to mobile hydraulic systems. They are also gaining popularity in the marine industry.

Hydraulic Oil Additives Specifications

We can alter the various properties of hydraulic oil by simply mixing it with various additives. The choice of additive will depend on the application conditions for which you want to use your hydraulic oil.

Here are some of the additives that you may come across:

· Viscosity Enhancers

As the name suggests, these additives are meant to enhance your oils viscosity index. This ensures that your hydraulic oil has a stable viscosity even when you expose it to any form of extreme temperature.

· Anti-wear Agents

When you have this additive in your oil, the oil will create a protective coating over any metal surface they contact. This coating can greatly reduce any wear and tear that occurs in your hydraulic system.

· Anti-Oxidants

This additive prevents your hydraulic oil from any form degradation brought about by oxidation. Oxidation generally reduces the shelf-life and performance of your hydraulic oil.

· Corrosion/Rust Inhibitors

They assist the hydraulic oil in preventing the formation of rust and any other form of corrosion in your hydraulic system.

· Anti-foaming Agents

With this additive in your oil, you can be sure that no kind of foam can be present in your hydraulic system. The impact of foam in your system is a reduction in the hydraulic pressure which can damage your hydraulic components.

· Detergents

You can use different detergents and dispersants in your oil to restrict the accumulation of contaminants. They can also prevent any build-up of depositions.

· Seal Swell Agents

These additives are blended with your hydraulic oil to reduce the rate at which hydraulic seals dry out. It assists in maintaining the integrity of the seals since it prevents them from breaking due to being brittle.

· Cold Flow

With this additive, you can comfortably use your hydraulic oil in applications having extremely cold weather conditions. It prevents your oil from freezing during operation due to the cold weather.

Viscosity of Hydraulic Oil

This is arguably one of the most important features that your hydraulic oil must have. We can describe the viscosity of hydraulic oil as its ability to resist various fluctuations in temperature.

It is recommended that you use a hydraulic oil that has a very high viscosity. Such oils are normally thick and very hard to compress and can therefore transmit the energy well.

We can measure the viscosity of hydraulic oil using the unit Centistokes (cSt). You can use an instrument called a viscometer to measure it at a temperature range of 40-100°C.

You have to include the temperature at which you measured your oil next to the viscosity value to give it clarity. This viscosity value is very important since a wrong viscous oil may reduce the efficiency of your equipment.

Hydraulic Oil Flash Point

This is the temperature that your hydraulic can be exposed to for it to emit combustible vapor. The combustible vapor released can generate a flash.

On a normal application, your hydraulic oil should have a flash point of 135°C. You should ensure that your oil does not exceed this temperature to prevent the risk of igniting it.

Hydraulic Oil Vs Hydraulic Fluid

Basic Parts of Hydraulic System
Basic Parts of Hydraulic System

People normally use these two words interchangeably and you might take them as the same thing. But in real sense, these two are quite different.

We already know that hydraulic oil is made from a mineral or synthetic base. This means that hydraulic oil is also a hydraulic fluid.

We can describe hydraulic fluid as any form of fluid that you use in a given hydraulic system. From this, we can conclusively say that hydraulic oil is a hydraulic fluid.

Temperature Range of Hydraulic Oil

Hydraulic oil can withstand either hot or cold temperatures. We have instances where applications have both hot and cold temperatures in their parameters.

Such conditions may rapidly degrade your oil and render it useless. However, with the precise blending of certain additives, your oil can perform efficiently despite the temperature variations.

One notable feature of hydraulic oil is its temperature stability that enables it to maintain its properties at certain temperatures. Any other temperature above or beyond that will render your oil unstable.

The effect of such exposure is your oil freezing in cold temperatures or a reduction in its viscosity leading to leakages in hot temperatures. Extremely high temperatures can rapidly degrade your oil.

There is a very close relationship between temperature and the viscosity of your hydraulic oil. Higher temperatures tend to reduce its viscosity while lower temperatures enhance it.

You can always blend your hydraulic oil with additives so that it can accommodate a wider operational temperature range. Such blenders ensure your oil can work in both low and high temperatures with equal efficiency.

Hydraulic Oil Density

When it comes to the lubrication efficiency of hydraulic oils, then its density plays a major role. Each hydraulic system has a specific density for the hydraulic oil that it can efficiently pump within its equipment.

This means that if you alter the density of that oil, then the efficiency of your whole hydraulic system changes. If you want to enhance how well your hydraulic system handles contaminants, then go for a high-density hydraulic oil.

They can effectively suspend the contaminants in their solution, transport them, and eventually eject them. This means that they can sustain contaminants during operation for longer before being removed by filters.

Depending on the intensity of the suspended contaminants, they have very little impact on your oil’s operation. Having a high-density hydraulic oil means that you can clean your system very easily.

Quality Levels of Hydraulic Oil

You will find hydraulic oil graded into various quality levels denoted by certain letters. Here are some of these quality levels you can find and what they entail:

Quality Levels of Hydraulic Oil
Quality Levels of Hydraulic Oil

· AW (Anti-wear)

This hydraulic oil has additives that have anti-wear properties. They reduce the rate of your equipment’s wear and prolong its service life.

· HM (High miscibility)

The miscibility level of such hydraulic oils is very high meaning you can easily blend it with other liquids. This is advantageous especially if there is are chance that fluids may mix in your application.

· HL (High lubricity)

This a hydraulic oil having very high lubricity levels. This enables the oil to regulate friction therefore ensuring that your component experiences limited wear and tear.

· HV (High viscosity)

This oil is very thick due to its high viscosity level. It can withstand extreme pressure forces and efficiently transmit the energy.

· SH (synthetic hydraulic)

These are hydraulic oils that are made of synthetic materials. They can be highly efficient in your application and are very clean.

Lifespan of Hydraulic Oil

Figure 6 Lifespan of hydraulic oil

The lifespan of your hydraulic is one of the main factors that you should consider when sourcing for it and replacing it. If your application environment is prone to dirt that may contaminate your oil, then its service life is reduced.

This means that you will have regular oil changes every time your hydraulic oil becomes heavily contaminated. You can prolong your oil service life by blending it with additives that counter the effects of contamination.

If you have a quality filtering system combined with desiccant breathers, then your hydraulic oil can stand the test of time. Additionally, you can incorporate appropriate technology with such contamination control practices to purify your oil.

You should also carry out frequent hydraulic oil analyses to regulate the frequency of your oil change. This will go a long way in saving you costs in terms of labor required and the cost of acquiring new oil.

When it comes to changing your hydraulic oil cooler, it is recommended that you do so after around 2000-3000 hours of active application. However, this will depend on your operating conditions and other factors such as:

  • Oxidation: when you use your oil for long, it will oxidize and form sludge. This may impact the performance of your equipment and requires immediate changing.
  • Insufficient Additives: the additives added to your oil may have lost their potency rendering their impact ineffective. This means that you need a hydraulic oil change.
  • Contamination: your hydraulic oil can have a build-up of contaminants such as dirt and water. These contaminants can lead to wear and tear in the hydraulic system meaning you will need an oil change.

Viscosity Index of Hydraulic Oil

We can describe the viscosity index of hydraulic oil as a measure of the change in viscosity due to any temperature changes.

The temperature changes have more effects when you’re dealing with hydraulic oil having a low viscosity index than a high viscosity index.

There exists a Viscosity Index Scale that has been formulated by the Society of Automotive Engineers (SAE). The scale shows various levels of viscosity at different degrees of temperature.

The scale grades the hydraulic oil as follows:

  • Low viscosity for a temperature range of 0-35°C
  • Medium viscosity for temperature range of 35°C to 80°C
  • High viscosity for temperature range of 80°C to 110°C
  • Very high viscosity for a temperature range of more than 110°C
Lifespan of Hydraulic Oil
Lifespan of Hydraulic Oil

Hydraulic Oil Analysis

If you want to get the most out of your hydraulic oil, then you need to carry out what we call hydraulic oil analysis. This condition-monitoring process is very vital before you decide to change your oil.

The procedure is very simple, you just take a small sample of the oil to a lab that critically analyses it. You will get a detailed report of its validity and whether you can continue using it or change it.

Such reports are important to any end-user since they stamp the usability of the oil. It also minimizes the damage that your equipment can incur from using worn-out or impure hydraulic oil.

We can list down more reasons why you need to carry out frequent hydraulic oil analysis:

  • Improves the performance of your equipment
  • Greatly reduces the risk of damage due to worn-out oil on your equipment
  • Saves you the price of frequently replacing your hydraulic oil

Hydraulic Oil ISO Grades

  • ISO 15: we normally use this hydraulic oil in hydraulic brake applications and power steering systems.
  • ISO 22:mostly used in the aviation industry to power airline equipment
  • ISO 32: this oil usually has a light viscosity and applied in light-duty hydraulic equipment. Such applications are best suited for low-viscosity hydraulic oil.
  • ISO 46: recommended for hydraulic applications that utilize hydraulic oil of medium viscosity. An example is the industrial plant equipment.
  • ISO 68: normally used in applications with system requirements of hydraulic oil having very high viscosity. You should note that such systems are usually high-pressure hydraulic units.
  • ISO 100: used mostly for high-viscosity applications such as industrial machinery handling heavy loads.
  • ISO 150: this type of oil is usually a special-purpose grade. It is commonly used in applications having elevated pressure and extreme temperature conditions.
  • ISO 220: used for applications that need hydraulic oil having a very high viscosity
  • ISO 320: used in heavy-duty applications. They are highly viscous
  • ISO 460: used in high-pressure applications that need hydraulic oil with a high viscosity
  • ISO 680:it is a highly viscous oil used in high-pressure applications
  • ISO 1000: highly viscous oil used in applications that rely on a highly viscous hydraulic oil
  • ISO 1500: it can withstand very low temperatures and has a very low viscosity. Mostly used in low-temperature applications that operate at high speeds.
  • ISO 22000: normally has medium viscosity and works very well in moderate temperature ranges. Commonly applied in systems having moderately low operational cycles.
  • ISO 32000: this oil can withstand extremely high temperatures and has a very high viscosity. Mostly applied in hydraulic systems having high load cycles and very high pressure.

Factors Affecting Performance Of Hydraulic Oil

You may have chosen the best hydraulic oil for your application but it is not giving you peak performance. There may be some factors that reduce the performance efficiency of your oil.

Let us have a look at some of them:

· Temperature

When you expose your oil without the appropriate additives to extremely high temperatures, it will rapidly degrade. Very high temperatures tend to reduce the viscosity of your oil therefore reducing its lubrication and efficiency.

On the other hand, subjecting your hydraulic oil to extremely low temperatures may lead to it waxing or even freezing. This can fail your equipment.

· Pressure

If you constantly exert too much pressure on your hydraulic oil, it will degrade very fast. The end result will be leakages in your hydraulic system and its eventual failure.

· Contamination

The contaminants we are talking about here are water, dust, or debris. When accumulated inside the hydraulic oil, they may reduce the efficiency and performance of the oil and your hydraulic system.

Choosing The Right Hydraulic Oil

Various hydraulic oils have different uses based on the additives that they have been blended with. Your choice of such will depend on how efficiently your hydraulic system will operate.

Let us look at some of the considerations you should take into account when sourcing for hydraulic oil:

· Hydraulic Oil Density

The density of hydraulic oil is its mass-to-volume ratio. High-density hydraulic oils usually tolerate contaminants well and can clean your system by removing them.

Don’t go for hydraulic oil having a high density when your system requires one with a lower density. This will greatly lower its efficiency in addition to damaging your equipment.

· Additives

Additives enable your hydraulic oil to withstand certain operational requirements of your system such as anti-wear or anti-oxidants. Go for hydraulic oil that contains additives that are specific to your application.

· Pump Design

The hydraulic oil in your system needs to be pumped to move all over the system. Each pump used usually requires oil to have a certain viscosity for it to appropriately flow.

Pumps such as vane and piston will pump oil having a different viscosity to one that can be pumped using gear pumps. Choose a hydraulic oil that corresponds with the type of pump used in your system.

· Thermal Stability

Hydraulic oils normally produce an acidic suspension when it reacts with heavy amounts of oxygen. This oxidation reaction tends to negatively impact its stability and lower its efficiency.

Always opt for hydraulic oil grades that are thermally stable. Also consider application factors such as water and contaminants that may interfere with the oxidation index of your oil.

Applications of Hydraulic Oil

You can use hydraulic oil in very many applications. Let us have a look at some insights on the various applications of hydraulic oil:

· Automotive Forklifts

The hydraulic oil powers the hydraulic system of the forklifts which enables them to lift very heavy loads.

· Aviation Industry

Aviation control systems heavily rely on hydraulic oil for their operations. The same oil is used in aircraft jacks and also in opening and closing hangar doors.

· Air Tools

Air compressors and any other air tool must have a type of hydraulic oil that can withstand high pressure. The anti-wear additives that have been blended in the oil offer protection to your air tools.

· Automotive Lifts

The automotive industry regularly uses car jacks and automotive lifts in their regular maintenance operations. All these equipment require a very reliable hydraulic oil for a safe and reliable operation.

These hydraulic jacks can lift very heavy vehicle loads without a lot of strain. That is why they require a hydraulic fluid with a very high viscosity that can work under high pressure.

· Tractors

Tractors have hydraulic brakes that make use of hydraulic oil besides other hydraulic system functions such as lifting and lowering ploughs. You need quality hydraulic oil to ensure the efficiency and protection of these expensive machinery.

· Marine Industry

Shipping vessels and cruise ships have stabilizers that are purely controlled using hydraulic oil. The function of these stabilizers is to reduce roll that normally alters the ship balance.


As we have seen, hydraulic oil is widely used across many industries and is very crucial in their commercial operations. It is for this reason that you should go for a hydraulic oil that has the highest quality and appropriate additives.

You can also use hydraulic oil as either a lubricant, coolant, or even as a sealant. Make sure that you go for a quality hydraulic oil that best suits your application conditions.

More Resources:

Oil for Hydraulic Systems – Source: WIKIPEDIA

Choosing Right Hydraulic Oil – Source: MACHINERY LUBRICATION

Hydraulic System Parts – Source: TARGET HYDRAULICS

Pressure Testing in Hydraulic Systems – Source: TARGET HYDRAULICS

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