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What is hydraulic oil?

Hydraulic oil, also known as hydraulic fluid, is a synthetic (man-made) or mineral-based, non-compressible fluid that is used to transfer power in hydraulic machinery and equipment.

Made up of oils and additives, hydraulic liquids transfer power whilst also acting as a lubricant and coolant. It works in a large range of temperatures and reduces wear, corrosion and rust.

What is the difference between mineral-based and synthetic hydraulic oil?

Mineral-based hydraulic oils are derived from crude oil fractions, whereas synthetic oils are manufactured using chemically produced base fluids.

Synthetic fluids can be formulated to impart superior physical properties as compared to mineral oils, such as high temperature performance, oxidation stability and biodegradability.

What is hydraulic oil used for?

As well as to transfer power, hydraulic oils also serve as a lubricant, coolant and sealant in machinery and equipment. They are used in pretty much every industry due to their large number of benefits; hydraulic application examples include:

  • Forklift trucks and stackers – to power the forks which lift heavy goods
  • Automated Production Machinery – used in a range of light, mid and heavy duty construction machinery such as cranes, excavators and tipper trucks
  • Wood (log) splitters – to power the ram mechanism on a hydraulic oil log splitter
  • Agricultural machinery and vehicles – to operate the hydraulic brakes and hydraulic systems such as boom arms
  • Aircraft – hydraulic oil must be reliable for control systems, aircraft hangar doors, aircraft jacks and aircraft controls
  • Automotive lifts – car lifts and car jacks require hydraulic jack oil to provide safety as well as performance
  • Nautical – for marine vessel stabilisers to reduce the amount of oil which can affect a ship’s balance and cause sea sickness
  • Snow ploughs – an essential component in the operation of the hydraulic lift, tilt and angle movements, and blended with anti-freeze additives
  • Construction – used in a range of light, mid and heavy duty construction machinery such as cranes, excavators and tipper trucks

Another common type of hydraulic oil is brake fluid which is used in hydraulic brake and clutch applications to transfer force into pressure and to increase braking force. Examples include passenger cars, motorbikes, light trucks and some bicycles which do not have liquids that are substantially compressible.

Brake fluids are usually glycol-ether based but silicone-based and mineral oil are also used.

Hydraulic oil additives

Depending on what you use hydraulic fluid for, additional additives may be needed to help its performance under different operating conditions. Additives include:

  • Anti-foaming – reduces foaming caused by detergents in the fluid, which can reduce the lubricating aspect of the fluid and cause machinery damage
  • Anti-oxidant – allows the oil to be used for longer and reduces sludge build-up
  • Anti-rust – forms a protective coating which reduces damage from rust and oxygen
  • Anti-wear – helps extend the lifetime of equipment (AW hydraulic fluids)

The above additives can be used on their own or in conjunction with other blends created for different functions. A hydraulic oil’s properties can be adjusted depending on which additives are used. Its general characteristics include a high viscosity index and being in-compressible.

Common uses of hydraulic oil and the type of additive(s) used to help optimum performance applications  


  High heat Heavy duty Cold weather
Examples Hot metal shears Coke furnace door openers Decanting machines Construction machinery
Offshore supply



Common problems Under high heat, oil becomes less viscous and flows easier which can cause leaks, rapid deterioration or loss of the required properties High-pressure environments will break machinery In arctic conditions, oil can start to solidify – if equipment is not properly lubricated it will seize up
Solution Additives retain their viscosity when exposed to high temperatures Anti-wear additives to cope under pressure Anti-freeze additives to avoid freezing or waxing. Low-temperature hydraulic oil is a popular name for fluid used here.

Is there a difference between hydraulic oil and hydraulic fluid?

Although hydraulic oil and hydraulic fluid are terms that are often used interchangeably, there are some subtle differences.

Hydraulic fluid has the same uses as hydraulic oil, however it is more commonly used in aircraft and automobile systems such as automatic transmissions, power brakes and steering.

Hydraulic oil ratings

Many end-users require ratings and approvals so they can rest assured that they’re using the correct oil for their equipment and machinery.

When a company sells an additive pack, they work with a manufacturer to produce an oil that works seamlessly with the manufacturer’s application. This will be traceable by a hydraulic oil supplier who has used the additive in the fluid.

Hydraulic oil grade list

  • AGMA – (American Gear Manufacturer’s Association, leading gear oil standards)
  • SAE – (Society of Automotive Engineers)
  • ISO VG – (International Standards Organisation). The higher the VG number the more viscous the fluid is. The VG number refers to which hydraulic oil is thicker, also known as the hydraulic oil weight

Grades with a W next to them indicate the weight (apart from automobile engine oil, which refers to winter oil). In the UK, ISO VG is used mainly for grading hydraulic oil.

A rough guide to common hydraulic oil ISO grades and their applications

  • ISO 15 Hydraulic Oil – ISO VG 15 Hydraulic Fluid is used in power steering and hydraulic brake systems
  • ISO 22 Hydraulic Oil – ISO VG 22 Hydraulic Fluid is generally used in airlines for air tools etc.
  • ISO 32 Hydraulic Oil – ISO VG 32 Hydraulic Fluid is ideal for use in high-powered machine tools
  • ISO 46 Hydraulic Oil – ISO VG 46 Hydraulic Fluid is normally required for industrial plant working under high-pressure
  • ISO 68 Hydraulic Oil – ISO VG 68 Hydraulic Fluid is designed for use in systems which require a large load-carrying ability
  • ISO 100 Hydraulic Oil – ISO VG 100 Hydraulic Fluid tends to be used in industrial machinery with heavy loads

Please note, some grades may cross over so it’s important to check with your supplier or manufacturer before use. Call us on 01432 367685 to find out which hydraulic oil is right for you.

Hydraulic oil properties

The properties of hydraulic oil are the reason why it’s so popular in commercial and industrial applications, and ensure a hydraulic system can work well under harsh operating conditions.

A good hydraulic oil has a combination of the following properties:

  • Non-compressible
  • Fire-resistant
  • Non-corrosive
  • Thermally stable in a range of temperatures
  • Anti-wearing
  • Unable to cavitate
  • Tolerant to water (resistant to water contamination)
  • Long life expectancy when stored correctly
  • Cost-effective
  • Viscous, regardless of temperature

There are very few oils that perfectly adhere to the above criteria. However, there are many hydraulic oils that are specialised for the environments in which they are required to operate.

Hydraulic oil classification

This refers to fluids that have different performance levels. Examples include:

  • HL: refined mineral oils with anti-rust and anti-oxidation properties
  • HM: the same properties as HL with improved anti-wear properties
  • HH: non-inhibited refined mineral oils
  • HR: HL oils with VI improvers

For more information, give us a call on 01432 367685

Hydraulic oil analysis

Also referred to as condition monitoring, it provides a detailed look at your oils to help you get the most out of them before switching your hydraulic application or damaging machinery.

At Arnold Engineering, we offer hydraulic oil analysis to all industries. We take a sample, analyse in the laboratory and provide you with a detailed report on whether you’re using the best oil for your application or whether an alternative would be better suited. This provides complete peace of mind that your expensive equipment is in safe hands and protected against dirty or worn oil.

If you’re a regular user of hydraulic oil, analysis should be high on your list. The benefits include:

  • Cost-effective solution to avoid unnecessary replacement
  • Reduces damage to machinery by identifying any issues early on
  • Enhanced life and performance of machinery and oil
  • Reduces risk of operating injuries and costs in claims, damages and downtime

Order your Hydraulic Oil Test kit here

Hydraulic oil flash point

Hydraulic oil flash point refers to the lowest temperature at which enough vapours are released from the fluid which are combustible. This is typically between 260 – 399°C. The lower the flashpoint, the easier it is to ignite.

They play a crucial part in commercial applications so it’s important that you purchase them from a reputable fuel supplier like Arnold Engineering. Get in touch today on 01432 367685 to find out more.

Hydraulic oil temperature range

Hydraulic oils have a wide temperature range. Depending on what it is being used for, it may endure both hot and cold temperatures which can render it useless if the oil hasn’t been blended with the right additives.

Hydraulic oils have temperature stability which simply means that they will maintain their properties within a particular temperature range. Anything outside of this range will have a negative impact on its temperature stability, causing the fluid to either wax or freeze in cold temperatures or lose viscosity and potentially leak in excessive heat.

Hydraulic oil viscosity

Viscosity is a key property of hydraulic oil and is the measurement of its resistance to flow. This means that it will resist compression at different rates depending on its viscosity and will take longer to pass through an orifice as the viscosity increases.

Hydraulic oil with a high viscosity will be thicker and harder to compress and move, whereas a low viscosity oil will be thinner and pass through much easier. The viscosity is measured in Centistokes (cSt) at temperatures of 40°C or 100°C in a laboratory setting using a viscometer. The value will always have the temperature next to it, as without this it’s meaningless.

The viscosity is key in many different applications – the wrong viscosity could make the equipment function poorly.

The viscosity and temperature of hydraulic oil are closely linked. As the temperature increases, the viscosity decreases – think of oil in a cooking pan; as the pan heats up, it becomes much more liquefied. Likewise, as the temperature decreases, the oil becomes more viscous.

Oil viscosity index

The viscosity index (VI) is used to measure a change in viscosity of hydraulic oil under changing temperatures. If it has a low viscosity index, a change in temperature will affect the viscosity more than if it has a high viscosity index.

Hydraulic oil with a high viscosity index is usually used in a hydraulic application with a greater range of ambient/operating temperatures.

A straight paraffinic mineral base oil tends to produce an oil with a low viscosity index, whereas one with viscosity improvers will produce a fluid with a high viscosity index.

The Society of Automotive Engineers (SAE) created a classification table (VI Scale) to show low to high viscosity levels depending on the temperature (°C). In the early days, the scale only went up to 100°C, however advances in hydraulic oil blending mean that the scale now goes much beyond this number.



Viscosity conversion chart




To understand the above chart, you need to read horizontally. It assumes 96 VI single grade oils, which is equivalent to viscosity at 40°C only.

Viscosity limits are simply a guesstimate; for more accurate data you should refer to your hydraulic oil supplier, ISO, AGMA and SAE specifications.

W grades are represented in terms of approximated 40°C viscosity. For low-temperature limits, you must check with SAE specifications.

How do hydraulic systems work?

The primary role of hydraulic oil is to transfer power from one end of a system to another end using a variety of hydraulic components.

An external force, such as a piston within a cylinder, is applied to the hydraulic oil which pushes the oil through the hydraulic system. Eventually, this produces a force on another part of the system which then results in a movement or action.

Please note that a key property of hydraulic oil is that it’s non-compressible, allowing for the most efficient transfer of force.

What is hydraulic oil made of?

Common hydraulic fluids are made up of one base fluid and then a number of additional ingredients which can be mixed depending on the desired properties required by your operations.

Examples include:

  • Esters
  • Mineral oil
  • Glycol
  • Silicone
  • Ethers

Hydraulic base oil

Hydraulic fluids date back to the ancient Egyptian times, when water was used as the base stock for many years. Then, during the 1920s, mineral oil was introduced and became a popular base oil due to its superior lubrication properties and its ability to be used at higher temperatures than the boiling point of water.

Today, many hydraulic oils are still based on mineral oil base stocks. However, with an increasing drive to use sustainable products, many natural oils such as rapeseed (canola oil) are now used as base oils to form biodegradable and renewable hydraulic fluids.

Additional base stocks can be used for specific applications, such as:

  • Glycol esters
  • Propylene glycol
  • Silicone oils
  • Organophosphate ester

How do you know if you’re using the right hydraulic oil?

When it comes to lubricated machines, there are many factors that need to be considered when selecting the right oil. A machine may run on a certain product; however, this doesn’t necessarily mean that this is the best product for the application. For optimum performance and longevity of your machines, you must carefully choose the right hydraulic oil.

Using the wrong oil can lead to an unexpected and severe reduction in operational efficiency which results in enhanced maintenance costs and regular disruptions.

With hydraulics, there are two primary considerations that you should keep in mind: the viscosity grade and hydraulic oil type (AW or R&O).

These factors are typically determined by the:

  • type of hydraulic pump employed by the system
  • operating temperature
  • system’s operating pressure

Is your hydraulic oil saving you money?

Regular maintenance of your hydraulic system can reduce performance problems and downtime, extending hydraulic component life and cutting operating costs.

Choosing low-priced hydraulic oil may save you money initially, but the consequence of low-quality oil will cost you much more in repair and machinery part replacement costs in the long run.

How to clean up hydraulic oil

It’s important to take great care when handling and storing hydraulic oil to avoid damaging the surrounding environment. We supply a range of hydraulic oil storage containers and ancillary products, including hydraulic pumps and drip trays to help contain any spills or leaks.

In the event of a larger spill, you will need specialist help from a professional company. If you experience a hydraulic oil spill, call our fuel experts as soon as possible on 01432 367685 to avoid huge clean up costs and prosecution.

Environmentally-friendly hydraulic oil

Biodegradable hydraulic oil is commonly used in applications where a spill or leak could contaminate the environment. The typical base oil in these fluids includes rapeseed oil and other vegetable oils which means that it will degrade naturally in the event of an accidental spill.

If you operate a farm, forest or environmentally sensitive site, environmentally-friendly hydraulic oil should be strongly considered for your operations.

Hydraulic oil price

As you can see, hydraulic fluids are an essential part of industrial and commercial applications. Pick the right hydraulic oil and the benefits add up; productivity gains and reduced overall operating costs. Arnold Engineering is here to deliver premium quality hydraulic oil in a range of quantities to businesses within 48 hours of purchase right from our base in Hereford.

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Wondering which hydraulic fluid to choose for your system? To learn more or request a price, call our knowledgeable team in Herefordshire today on 01432 367685

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