Understanding Hydraulic Fluid Types

Hydraulic Fluid Types

A mechanic adds hydraulic fluid to a vehicle's brake system.

Figure 1: A mechanic adds hydraulic fluid to a vehicle's brake system.

Hydraulic fluid is important in various industries, such as construction, manufacturing, aviation, and marine applications. With numerous hydraulic fluid types available, it can be difficult to understand which type to select for a particular piece of equipment. This article covers the different types of hydraulic fluid, their characteristics and applications.

Table of contents

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What is hydraulic fluid?

Hydraulic fluid is a non-compressible, engineered medium that provides lubrication and heat and power transfer properties. It is composed of a base oil, which can be mineral or synthetic, along with various performance-enhancing additives such as anti-wear agents, viscosity improvers, rust and corrosion inhibitors, and anti-foaming agents.

Hydraulic fluid's ability to transmit force and motion is due to its incompressible nature, meaning it cannot be compressed or easily deformed under pressure. Force applied to the fluid at one end of the system creates pressure, which transmits through the fluid to the other end of the system. This pressure can generate movement or force by actuating hydraulic cylinders, motors, or valves.

Furthermore, the hydraulic fluid's viscosity and flow properties play a vital role in transmitting power. The fluid's viscosity affects its ability to flow through narrow channels and around sharp corners, while its flow rate determines the speed at which pressure is transmitted through the system.

Hydraulic fluid types

There are different types of hydraulic fluids. Mineral-based fluids are common and inexpensive, while synthetic fluids perform better under extreme conditions. Water-based fluids are safe around fire and biodegradable fluids are better for the environment.

Mineral-based hydraulic fluid

Mineral-based hydraulic fluid is made from refined petroleum or mineral oil. This type is most commonly used due to its low cost and availability. It has good lubrication properties and can handle high temperatures, but it is not biodegradable and can be flammable.


Mineral-based hydraulic fluids are often used in industrial and mobile equipment, such as hydraulic presses, construction equipment, and agricultural machinery. However, they are not suitable for use in high-temperature environments or applications where water contamination is a concern. In such cases, synthetic hydraulic fluids are a better choice.

Synthetic hydraulic fluid

Synthetic hydraulic fluid is made from chemically synthesized base oils, including polyalphaolefins, esters, and other synthetic compounds. Synthetic fluids offer improved performance in extreme temperatures and high-pressure applications for the following reasons:

  • Higher viscosity index: They maintain their viscosity better over a wide range of temperatures. This allows them to provide consistent performance in extreme temperatures, whether hot or cold.
  • Oxidation resistance: They are less prone to oxidation. Oxidation can break down the fluid, reducing its effectiveness and potentially damaging the hydraulic system. Synthetic fluids are more stable and can withstand the high temperatures generated by high-pressure applications.
  • Shear stability: They are more shear stable. Shearing occurs when the fluid is subjected to high pressure, causing it to break down and lose its viscosity. Synthetic fluids resist this breakdown.


Synthetic hydraulic fluids are designed for heavy-duty industrial machinery and equipment that operate under high temperature and pressure conditions in harsh environments. They provide superior viscosity stability, oxidation resistance, and wear protection, making them ideal for critical applications such as aerospace systems, injection molding machines, and marine equipment. Compared to mineral-based fluids, they have a longer service life and are less prone to degradation. However, it is important to select synthetic hydraulic fluids based on the specific needs of the system, as they are not suitable for all applications.

Water-based hydraulic fluid

Water-based hydraulic fluids are mixtures of water and other additives such as glycols, surfactants, and corrosion inhibitors. They are a fire resistant hydraulic fluid used in applications where fire safety is a concern. They are also biodegradable and have low toxicity, making them suitable for environmental applications. However, water-based hydraulic fluids are less effective in cold temperatures.


Water-based hydraulic fluids are used in high-risk industries such as metalworking and mining, where fire hazards and environmental impact are important considerations. These fluids offer good heat transfer properties and are effective in dissipating heat in high-temperature applications. Despite being environmentally friendly, they can be corrosive to some materials and may require additional additives to prevent rust and corrosion. Moreover, they are not suitable for applications that need high lubricity or in systems that are vulnerable to water contamination.

Biodegradable hydraulic fluid

Biodegradable hydraulic fluids break down naturally in the environment and are typically made from vegetable oils or synthetic esters. They are essential for applications where spills or leaks could have a significant impact on the environment, such as marine or forestry equipment. However, biodegradable fluids can be more expensive and have lower performance characteristics.


Biodegradable hydraulic fluids are for applications where environmental impact and sustainability are crucial. They are especially used in sensitive areas such as national parks, forests, and waterways to prevent harm to the environment. These fluids are typically made from renewable sources like vegetable oils and break down naturally over time. They offer good lubrication properties and are suitable for mobile equipment like tractors and forestry machinery. However, they are not as effective as traditional hydraulic fluids and may require more frequent replacement.

Selecting hydraulic fluid

Select a hydraulic fluid based on the application factors:

  • No fire risk: Mineral-based hydraulic fluid is the most cost-effective choice. However, do not use it in applications that have a risk of fire.
  • Fire risk: For applications that have a fire risk, consider choosing water-based hydraulic fluids.
  • High temperature and/or pressure: For high temperature and/or pressure applications, choose a synthetic hydraulic fluid.
  • Environmental concerns: If avoiding damage to the environment is a key concern, look into water-based and biodegradable hydraulic fluids.

Hydraulic fluid characteristics

Hydraulic fluids have the following characteristics to consider when choosing one for an application.

  • Anti-wear properties: Without the right additives, hydraulic fluid cannot stop pumps from prematurely wearing out and can lower the system's pressure. Machines with powerful pumps especially need good anti-wear properties to work well.
  • Density: A fluid's density affects how well it can lubricate and perform. Hydraulic systems have specific requirements, so machines only work well with certain densities. If the density changes, the efficiency of the equipment will be affected.
  • Fire resistance: Fire resistance refers to the fluid's ability to minimize the risk of ignition or combustion, which is important in high-temperature or hazardous environments.
  • Lubricity: Lubricity is the fluid's ability to lubricate moving parts and reduce friction, which can affect the efficiency and lifespan of the hydraulic system.
  • Oxidation stability: Oxidation stability refers to the fluid's ability to resist chemical reactions with oxygen over time, which can cause it to break down or become less effective.
  • Temperature range: A hydraulic fluid's temperature range is the range of temperatures at which it can operate effectively without breaking down or losing its properties.
  • Shear stability: Shear stability is the fluid's ability to maintain its viscosity and other properties under high shear or stress conditions, such as those encountered in pumps or hydraulic systems.
  • Viscosity: Viscosity is the thickness or resistance to flow of the fluid. Viscosity is measured in units of centipoise (cP) or Pascal-seconds (Pa-s).
  • Water resistance: A hydraulic fluid's water resistance is the fluid's ability to resist or repel water, which can affect its performance in wet environments or when exposed to moisture.

Other considerations

  • Manufacturer: The best source to find the right hydraulic fluid for a machine is the manufacturer's manual.
  • Mixing: Mixing different hydraulic fluids can cause reduced performance and system failure.
  • Monitor and maintain: Hydraulic fluid will degrade over time and with use. Regularly monitor the fluid and perform regular maintenance, including replacing the fluid when necessary.

Hydraulic fluid substitutes

Various alternatives help machine operators and owners keep hydraulic systems lubricated, even when recommended fluids are depleted and replacements are needed fast. Most are cheaper and easily accessible. They include:

  • Motor oil: Engine oil can work well in hydraulic systems and heavy machine parts because it's viscous. However, the same property makes it unsuitable for extended use because high-viscosity fluids often damage hydraulic systems.
  • Brake fluid: Brake fluid shares several properties with hydraulic oil, including optimum viscosity. Therefore, it can be a suitable substitute when machines run out unexpectedly.
  • Automatic transmission fluid: Automatic transmission fluids are among the most popular hydraulic fluid substitutes because they have an extensive operating temperature range and can maintain optimal performance in diverse environments.
  • Vegetable seed oils: Most vegetable oils have higher flash and fire points. Plus, they have good lubricating properties. These characteristics make vegetable seed oils ideal substitutes for hydraulic fluids, more so in systems that demand a reasonable margin of safety against fire outbreaks.
  • Water: Water has low compressibility, making it an excellent hydraulic fluid substitute. Plus, it has unrivaled fire-resistance properties. Do not use water in machines without corrosion resistance.

Only use substitutes when hydraulic fluid is not readily available. Do not be over-reliant on the above alternatives because some can encourage corrosion, overheating, and aggravate wear in machine parts.


Is all hydraulic fluid the same?

No. Different options are available, including water-based, mineral-based, and synthetic fluids. Most have different chemical makeups and unique characteristics, including viscosity, anti-wear additives, and recommended operating conditions.

Are hydraulic oils different from hydraulic fluids?

Hydraulic oil and hydraulic fluid are often used interchangeably for the right reasons. Hydraulic oils are hydraulic hydrostatic fluids, meaning they are products used in hydrostatic applications.

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