Fluid in motion

Fluids, or hydraulic oils are essential to your hydraulic system. They have the important role of transferring energy within the system. Hydraulic fluids remove extra heat from the system, help get rid of contaminants, and prevent parts from wear.

Choose your hydraulic fluid wisely. The wrong fluid for an application can reduce its service-life, produce extra heat, and leave tar and other deposits behind.

Types of hydraulic fluids

There are various types of hydraulic fluids. Although, they can be categorized into three buckets: petroleum based, water based, or synthetic. Each type has a different purpose and the one you select will depend on the application.


Petroleum-based fluids are made from a refining crude oil process. Most commonly used, petroleum-based oils are considered a standard type of hydraulic fluid. They are an affordable and quality option.


Water-based fluids are mainly used when there is a risk of fires starting. However, these fluids can produce a lot of excess heat for the system. Water-based fluids are a costly option and do not have a good parts wear resistance.


Synthetic or man-made fluids are put together with a specific performance goal in mind. They are a good selection when temperature or pressure levels are at a low or high extreme. However, these fluid types are the costliest out of all the fluid types. They also can be toxic. 

Consider the key factors

When selecting hydraulic fluid for your system there are several key factors to keep in mind. These factors are viscosity, viscosity index, wear resistance, oxidation stability, and lubrication.


Viscosity represents the thickness of a fluid. The viscosity grade will impact how well the system as a whole runs.

When the grade is too low, there is a higher likelihood of wear on the parts from metal-on-metal contact. When the grade is too high for the system, the fluid has more resistance to flow. This can produce extra heat. Making sure that the viscosity is at the right level for your system is essential. The wrong viscosity level can cause damage to the system. To determine your viscosity grade, know the standards for your pump type.

Viscosity Index

Viscosity index is how the viscosity changes when there is a fluctuation in temperature. The higher the index, the more the viscosity will stay the same across a large temperature range. Petroleum based fluids typically have an index of 90 to 105. Synthetic based fluids can reach an index of up to 160. 

Wear Resistance

Wear resistance is how much the hydraulic fluid can decrease the wear that is caused by parts rubbing against each other.

Oxidation Stability

Oxidation stability is the fluid’s ability to resist the deterioration oxidation causes. Oxidation involves a chemical reaction between the hydraulic fluid and oxygen. When this happens, the viscosity level is increased—causing the fluid to become more resistant to flow.

This chemical reaction can drastically decrease the service-life of your hydraulic oil. According to the Arrhenius rate rule, per every time the temperature goes up by 18OF after 140OF, the life of your hydraulic fluid is cut in half. Temperatures below 140OF have the most stability. To figure out how stable your fluid is against oxidation, find the operating temperature range of your system provided by the manufacturer.


Lubrication goes along with wear resistance. The hydraulic fluid should have a film that decreases the friction levels.

Make your choice

Know the facts

In order to select the correct fluid type, you will need to compare the fluid’s specs to the pump’s requirements. You should compare viscosity grade, pressure range, operating temperature of the pump and fluid to see if they are a good match. Piston pumps, vane pumps, and gear pumps are the three main types. Each pump has their own requirements outlined by the manufacturer. However, generally their information is as follows:

Piston Pumps

Piston pumps have a viscosity range of 10 to 160 centistokes (cSt). When running, piston pumps have a pressure rating of up to 6000 psi. This pump type has a higher resistance to wear than other pumps.

Vane Pumps

Vane pumps have a viscosity grade ranging from 14 to 160 cSt.  These pumps do not perform well with high pressures. Having a fluid with good wear resistance is essential for this type of pump.

Gear Pumps

Gear pumps can be classified as either internal or external. Both classifications have pressure levels of 3000 to 3500 psi. Internal gear pumps can reach viscosity up to 2,200 cSt. External gear pumps have viscosity grades up to 300 cSt. Internal gear pumps are typically more efficient pump types and have the capability to operate at low viscosity grades.

Evaluate the application

After you have compared the pump’s requirements to the fluid’s properties, consider the application. This is the most important step in choosing a hydraulic fluid. Determine if it is worth purchasing a high-quality fluid. If your system is subject to a lot of contamination or leaks, you won’t want to purchase the best of the best fluid available.

Think about what makes the most sense for your system, budget, and safety. When in doubt, you can always contact your manufacturer for their hydraulic fluid recommendation.

Add to your hydraulic fluid

Looking to add more to your hydraulic fluid? Additives can enhance your hydraulic fluid’s performance. Additives are either solids or liquids that are put into the hydraulic fluid. Their main purpose is to adjust the fluid itself for specific purposes. Anti-oxidants, rust and corrosion inhibitors, viscosity index improvers, and anti-wear agents are just a few examples of additives. Each of these has a specific benefit to the system. Although, if there is too much of an additive it can do more harm than good.

Industrial Hydraulics offers a variety of hydraulic fluids</a > that are of high quality and value to your hydraulic system. Call us at 1-800-722-6792 for more information today.