Most hydraulic systems can operate using many different fluids, including multigrade engine oil, automatic transmission fluid and more conventional antiwear (AW) hydraulic oil. Which oil or fluid you choose depends on your equipment and how you plan to use it.

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While it is not possible to make one definitive recommendation that covers all types of hydraulic equipment in all applications, there are a few key questions to ask yourself when selecting a hydraulic fluid.

What is the Right Viscosity Grade?

Picking the right viscosity grade (sometimes referred to simply as the "hydraulic fluid grade") is the single most important factor when selecting a hydraulic oil or fluid. It doesn’t matter how good the other properties of the oil are if the viscosity grade is not matched to the operating temperature range of the hydraulic system where it will be used.

If you choose a viscosity grade incorrectly, your hydraulic components will degrade faster than they should.

How to Choose the Right Hydraulic Oil Viscosity or Grade

To choose the right  fluid viscosity grade for your particular system, you will need to consider:

• starting viscosity at minimum ambient temperature

• maximum expected operating temperature, which is influenced by maximum ambient temperature

• permissible and optimum viscosity range for the system’s components

Typical minimum permissible and optimum viscosity values for different types of hydraulic components are shown below.

Table 1. Typical Minimum Viscosity Values for Hydraulic Components
  Component Type Minimum Permissible
Viscosity (cSt) Minimum Optimum
Viscosity (cSt) Vane 25 25 External Gear 10 25 Internal Gear 20 25 Radial Piston 18 30 Axial Piston 10 16

When to Use a Multigrade Hydraulic Oil

If the hydraulic system is required to operate in freezing temperatures in winter and tropical conditions in summer, then it will likely need multigrade oil to maintain viscosity within permissible limits across a wide operating temperature range.

If fluid viscosity can be maintained in the optimum range, typically 25 to 36 centistokes, the overall efficiency of the hydraulic system is maximized (less input power is given up to heat). This means that under certain conditions, the use of a multigrade can reduce the power consumption of the hydraulic system.

For mobile hydraulic equipment users, this translates to reduced fuel consumption.

There are some concerns when using multigrade fluids in hydraulic systems. The viscosity index (VI) improvers used to make multigrade oils can have a negative effect on the air separation properties of the oil.1

This is not ideal, particularly in mobile hydraulic systems which have a small reservoir with poor deaeration characteristics. The high shear rates and turbulent flow conditions often present in hydraulic systems destroy the molecular bonds of the VI improvers over time, resulting in loss of viscosity.

When selecting a high VI or multigrade fluid, it is recommended that the hydraulic component manufacturers’ minimum permissible viscosity values (Table 1) be increased by 30 percent to compensate for VI improver sheardown. This adjustment reduces the maximum permissible operating temperature that would otherwise be allowable with the selected oil, thereby providing a margin of safety for viscosity loss through VI improver shearing.

When to Use a Monograde Hydraulic Oil

If the hydraulic system has a narrow operating temperature range and it is possible to maintain optimum fluid viscosity using a monograde oil, it is recommended not to use a multigrade for the reasons stated above.

Use these factors, along with other viscosity considerations, to make the best choice for your needs.

Should My Hydraulic Fluid Use a Detergent?

DIN ; HLP-D fluids are a class of antiwear hydraulic fluids that contain detergents and dispersants; the use of these fluids is approved by most major hydraulic component manufacturers. Detergent oils have the ability to emulsify water and disperse and suspend other contaminants such as varnish and sludge.

This keeps components free from deposits, but it also means that contaminants do not settle out — they must be filtered out. These can be desirable properties in mobile hydraulic systems, which, unlike industrial systems, have little opportunity for the settling and precipitation of contaminants at the reservoir, due to its small volume.

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The main concern with these fluids is that they have excellent water emulsifying ability, which means that if present, water is not separated out of the fluid. Water accelerates the aging of the oil, reduces lubricity and filterability, reduces seal life and leads to corrosion and cavitation.

Emulsified water can be turned into steam at highly loaded parts of the system. Avoid these problems by maintaining water content below the oil’s saturation point at operating temperature.

Should I Use Antiwear (AW) Hydraulic Fluid?

The purpose of antiwear additives is to maintain lubrication under boundary conditions. The most common antiwear additive used in engine and hydraulic oil is zinc dialkyl dithiophosphate (ZDDP).

The presence of ZDDP is not always seen as a positive because it can chemically break down and attack some metals and reduce filterability. Stabilized ZDDP chemistry has largely overcome these shortcomings, making it an essential additive to the fluid used in any high-pressure, high-performance hydraulic system, such as those fitted with piston pumps and motors.

In today’s market, however, zinc-free anti-wear hydraulic oils have emerged as a versatile and environmentally friendly alternative to traditional zinc-based formulations. These advanced lubricants provide excellent wear protection for critical hydraulic components without relying on zinc-based additives like ZDDP. Instead, they utilize ashless or phosphorus-based chemistries to deliver comparable or superior performance, including exceptional thermal stability, oxidation resistance, and system cleanliness.

Hyperion™ Synthetic Blend oils are zinc-free, anti-wear, hydraulic oils meet the needs of today's high precision hydraulic systems, turbines and air compressors. 

Zinc-free oils are particularly valuable in applications where environmental regulations are stringent or where zinc additives could cause compatibility issues with modern elastomers and seals. With growing demand for sustainable and high-performing solutions, zinc-free anti-wear hydraulic oils represent a significant step forward in lubrication technology, meeting the needs of industries that require both efficiency and eco-consciousness.

Do You Understand the OEM Recommendations?

If you have considered all of the above questions, it is a good idea to review the OEM's (original equipment manufacturer's) specifications.

There may be a warranty-related reason to follow the equipment manufacturer’s recommendations. However, in some applications, especially those with extreme temperatures or other unusual operating conditions, using something different than what the manufacturer recommends may increase hydraulic system performance and reliability. For example, Whitmore uses an ashless (zinc-free) additive system that further enhances the longevity and high temperature performance of the Hyperion™ Synthetic-Blend series.

If you do not have the proper training to make such a decision, it's a good idea to talk with a technical specialist, preferably one that is vendor-neutral, because this will truly give you the best recommendation for your needs.

Read more on hydraulic best practices:

How Do You Know if You're Using the Right Hydraulic Oil?

The Seven Most Common Hydraulic Equipment Mistakes

Hydraulic Filter Location Pros and Cons

How to Combat Leaking Hydraulic Connections

Reference

1. Mannesmann Rexroth. “Mineral Oil-based Pressure Fluids for Vane Pumps, Radial Piston Pumps and Gear Pumps as Well as GM, GMRP, MCS, MCR, MR and MKM/MRM Motors.” (RE 07 075/07.98), p.2. .

Everything You Need to Know About Hydraulic Oil

You need to consider various factors to determine what type of hydraulic oil is best for your hydraulic system. A hydraulic machine manufacturer could recommend one type of oil. However, this does not mean it is always the best oil, depending on how you use the machine and in what operating conditions.

Using the wrong type of hydraulic fluid can lead to inefficiencies, system breakdowns, system failures, and increased maintenance costs. To get the longest life out of your hydraulic machinery and related parts and components, you need to select the most appropriate hydraulic fluid.

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What Is Hydraulic Oil?

Hydraulic oil, or hydraulic fluid, is used in hydraulic systems and machinery to transmit power by pumping the non-compressible fluid through the system to generate pressure that is converted into force to complete desired tasks and work. Hydraulic oils can be oil-based mineral oils, synthetic-based, or water-based.
 
There are two primary factors you need to remember when selecting hydraulic oils:
 

1. The Viscosity of the Oil
2. The Type of Hydraulic Oil

What Are Some Uses for Hydraulic Oil?

Even though hydraulic fluid is primarily used in hydraulic systems and hydraulic machinery, there are other uses for it, including:
 

• As a lubricant to help keep moving parts and components operating correctly and reducing friction.
• As a coolant to absorb heat from various areas inside the system, and then release the heat in another area.
• As a sealant to help prevent leaks.

Why You Need to Understand the Flash Points of Hydraulic Oil

All hydraulic fluids have an ignition temperature you need to be aware of when selecting the fluid to use in a hydraulic system. Hydraulic fluids can become very hot as they pass through the system. While there is some heat transfer through exposed hydraulic hoses and pipes, it still does not reduce all the risks associated with extremely hot hydraulic fluid.
 
Oil-based fluids have a flash point temperature range from around 150 – 300 degrees Celsius.  Depending on the type of fluid used, it can catch fire when it is 250 – 375 degrees Celsius.
 
However, the ignition temperature of hydraulic fluid can be lower, especially when it is under high pressure and accidentally sprayed from a burst hydraulic pipe, fitting, coupling, or other components onto hot surfaces. The mist of oil can easily ignite and start a fire.
 
By being aware of the fire risks and flash points of hydraulic oil, you can determine which type of fluid will best meet your needs and reduce fire risks.
 

Why Oil Viscosity and Operating Temperatures Matter

The viscosity of hydraulic fluids changes as temperatures increase or decrease. As temperatures increase, the oil becomes thinner. Conversely, as temperatures decrease, the oil becomes thicker. So, in cold environments, a lower viscosity fluid would be better because it is thinner and will not cause flow issues.
 
On the other hand, a higher viscosity fluid is better in warmer environments because it is thicker. As the system reaches a good viscosity operating temperature, the fluid will not become too thin.
 

How to Choose the Right Hydraulic Oil

Choosing the right hydraulic oil for your hydraulic system requires knowing what the recommended fluid type and viscosity are for the system. The manufacturer of the hydraulic pumps used in the system will recommend an oil viscosity.
 
A good starting point is using the recommended hydraulic fluid for your hydraulic pumps. Using this information, review the following factors to make adjustments to the type of hydraulic oil you need:
 

• The System Operating Pressure – What are the minimum and maximum pressures of your system?
• The Operating Environment – Is the system used indoors or outdoors? Are there dry and dusty conditions? Are there large variations in the ambient temperature ?
• Heat Transfer Rates – How much heat transfer is there between the hydraulic oil and system parts and components?
• Fire Risks – Would hot surfaces pose a fire risk if a hydraulic hose burst and fluid landed on them? Are there other fire risks you need to consider?
• Environmental Considerations – Is the system operating in an environment where a spill would be difficult to contain and potentially be detrimental to the natural world?

Taking all of these things into account, you can then determine the most appropriate type of hydraulic fluid for your hydraulic system.
 

Water-Based Hydraulic Oil

While it is one of the least used fluids, this is perfect for environments where the fire risks are extremely high. Water glycol is used to create a fire-resistant hydraulic fluid. However, you need to make sure the hydraulic pumps and other hydraulic parts and components are compatible with water-based hydraulic fluids due to their corrosive effects.
 

Synthetic Hydraulic Oil

Full synthetic hydraulic fluid is becoming more popular in newer hydraulic systems. The oil tends to last longer before it starts to degrade and needs to be replaced.

Another benefit of synthetic hydraulic fluid is it can handle temperature extremes from cold winter days to hot summer days. Additionally, it is well-suited to hydraulic systems that need to be operated at high pressures continuously.

Petroleum-Based Hydraulic Oil

Petroleum-based hydraulic fluid is the most common type of fluid used. It provides flexibility for a wide range of hydraulic systems that are operated in different conditions and environments.
 
Petroleum fluids are blended with various additives to help extend the life of parts, components, and seals by reducing wear and tear on them, preventing rust and oxidation with corrosion protection, and improving system performance.
 
As you can see, there are several considerations you must account for when choosing the most appropriate hydraulic oil for your hydraulic system and machinery. If you need help deciding which fluid is best or if you need hydraulic pumps, motors, cylinders, and other parts, please feel free to contact White House Products, Ltd. at +44 (0) today!
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