Hydraulic Oil – The Critical Component

Hydraulic systems are employed in a multitude of applications, operating in a wide-variety of working conditions. Consistent system performance relies on many factors, but one of the biggest contributors is the hydraulic oil. Hydraulic oils are a critical system component, but their importance is generally understated and they are wrongly considered to be commodities.

Functions of a Hydraulic Oil

Within a hydraulic system, the hydraulic oil has four main important functions:

·        To act as an energy transfer medium

·        To lubricate and protect system components

·        To transfer and dissipate heat

·        To seal small clearances between moving parts

To effectively deliver these functions, hydraulic oils have to be carefully formulated with high-quality base fluids and performance additives.


In the main, hydraulic oils tend to be referred to by their viscosity (or thickness), i.e. 32, 46 or 68.

These are the most common ISO classification grades, although there are thinner or thicker alternatives, depending on the machine’s requirements.  However, this only tells part of the story and not every 32 grade, for example, is the same as another. Providing the oil has been manufactured by a reputable company it will also be expected to have many other performance features to fully protect the equipment.

Operating temperatures

As an energy transfer medium, it has to perform consistently over the equipment’s operating temperature and load range, allowing precise control when needed.  Hydraulic oils formulated using inferior base fluids have a limited temperature range and at high temperatures they lose their viscosity (or thickness) making it difficult for the pump to push the oil around the system.  The effectiveness of a pump is referred to as its volumetric efficiency which will decrease if the oil thins down too dramatically.  Well formulated hydraulic oils, using quality base fluids, minimise this affect as temperature increases.

Where equipment is operating in applications with extremes of temperature, high performance hydraulic oils are required to cope with these varying conditions.  This is achieved using polymer technology that allows the oil to flow at low temperatures, but delivers effective working pressure when the equipment is hot and working hard.

Component Protection

The heart of any hydraulic system is the pump and this is where correct lubrication is absolutely essential.  Common designs include rotary vane, gear and variable displacement piston pumps.

Each design has specific lubrication requirements and hydraulic oils are formulated to address these various operational demands:

1.      Rotary vane: Powerful anti-wear chemistry protects the sliding vanes and bearings and provides an oil film to seal the vanes against the cylinder.  This sealing ability ensures the hydraulic oil itself is compressed to provide the required discharge pressure.

2.      Gear (Internal and external): Prevents the gear teeth from wearing, seals the space between the gear teeth and cylinder.  Protects bearings.

3.      Variable displacement piston pump:  These pumps are extremely complex, with various metal types that require a high level of anti-wear chemistry. Sealing of the pistons is also required to deliver the required working pressure.

Hydraulic oils are also treated to ensure they can withstand high temperatures for prolonged periods of time without breaking down and causing deposits in the system.  They will also have the ability to combat rusting or corrosion of components which can occur due to the ingress of airborne moisture or the working environment.  Mechanical churning of the oil can cause foaming which in turn results in ‘sponginess’ and poor hydraulic response and accelerated wear. To prevent this anti-foam additives are also incorporated.


Pumps, control valves and pressure relief valves are all machined with fine tolerances and are therefore susceptible to contamination within the system.  Harmful solid matter can be removed safely by a filter or filters, however, even contamination by particles invisible to the naked eye can cause damage and this can be introduced from new oil.  New oil does not necessarily mean clean oil and its important that any oil is supplied having been correctly filtered at the point of manufacture and into clean and dry containers or bulk delivery units.

Main types of Hydraulic Oil

Hydraulic oils can be formulated with different base oil types depending on the final application.  In general, mineral oil based formulations, with a correctly balanced additive system, form the majority of products on the market.  But variations do exist to provide a tailored approach.

If fire resistance is required there are families of base oils that are non-combustible, or formulations that are self-extinguishing, i.e. they may contain water.  Biodegradable options are also available, either based on vegetable oils or synthetic esters that mimic vegetable oil, where environmental impact has to be taken onto consideration.  In some specialised applications, food safe maybe specified and again the appropriate base oil has to be selected.

At Morris Lubricants, we have a wide variety of hydraulic oils, which can be found here.


In order to ensure full compliance with the systems requirements it also worth noting the specifications relevant to this industry.  The main pump manufacturers have their own performance levels or specifications (Bosch, Denison, Parker, etc.) and there are several general industry specifications (DIN, US Steel, ISO, etc,) that end users should be aware of.  As with any equipment, always ensure that an oil with the correct viscosity grade and specification is chosen, as issued by the manufacturer, for the highest level of protection and system efficiency.

In summary, not all hydraulic oils are the same, even if some of the basic qualities appear equal, such as viscosity.  Price shouldn’t be the only factor when choosing a hydraulic fluid as it can be a false economy that may lead to expensive repairs and downtime, further down the line.

Blog by Adrian Hill, Automotive Product Manager at Morris Lubricants.

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