How to Choose the Right Hydraulic Fluid (or Hydraulic Oil)

Brendan Casey

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.

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 51524; 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.

 

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.

A ZDDP concentration of at least 900 parts per million can be beneficial in mobile applications.

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 extremes of temperature or other unusual operating conditions, using something different than what the manufacturer recommends may increase hydraulic system performance and reliability.

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. 1998.

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About the Author

Brendan Casey has more than 20 years experience in the maintenance, repair and overhaul of mobile and industrial equipment. For more information on reducing the operating cost and increasing the...