Heavy-Duty Hydraulic Oil Grades Explained

Heavy-Duty Hydraulic Oil Grades Explained

Feb 28th 2023

Choosing the right hydraulic oil for your application is critical in order to protect and prolong the lifespan of your equipment. However, to select the correct hydraulic fluid for your application, you must first understand how they’re graded. To help, let’s take a deep dive into heavy-duty hydraulic oil grades.

How Hydraulic Oils Get Graded

Hydraulic oils are primarily graded by their weight and kinematic viscosity, with viscosity being the most important defining characteristic. Kinematic viscosity measures a fluid’s velocity, or rather, its resistance to flow under gravitational forces, and how resistant these fluids and their properties are to temperature changes. The higher the temperature is, the lower the viscosity, or the “easier” the fluid flows, and vice versa.

When looking at their viscosity index, oils get placed into two viscosity grades: monograde and multigrade. Monograde oils get categorized by their ambient temperatures in heat and cold. Multigrade oils contain additives that allow them to remain at a particular viscosity during various temperatures.

Hydraulic Oil Classifications

These different viscosities get measured, tested, and classified through various organizations, all with different systems. Depending on the application, your oil may have to meet either the International Organization Standardization (ISO), Society of Automobile Engineers (SAE), or American Gear Manufacturer’s Association (AGMA) standards. These classifications are technically interchangeable–for example, ISO VG 100 is equivalent to AGMA 3 or SAE 30–but it’s best to consult OEM specifications to be on the safe side.

Reading the Equivalency Chart

The viscosity equivalent chart allows you to look at the different grades of hydraulic oil and their classifications, as well as their kinematic viscosity and Saybolt viscosity.

The middle of the chart contains four categories: ISO VG, AGMA Grade, SAE Engine Oils, and SAE Gear Oils. On the left and right side, respectively, are the kinematic viscosity measured in Centistokes (cSt) and Saybolt Viscosity in Saybolt Universal seconds. Saybolt Viscosity is generally considered obsolete, so most of the time, you won’t have to encounter this measurement.

Centistokes are one-hundredth of a stoke and are a unit of viscosity that measures the density of the fluid as well as the viscosity. The higher the fluid’s cSt measurement, the slower said fluid is. Directly to the right of this measurement, you’ll see its equivalent classification under ISO AGMA and SAE standards. For example, when looking at the chart, you’ll notice an oil with a kinematic velocity of 200 will be equivalent to 220 ISO VG oil, 5 AGMA oil, or a 50 SAE Engine Oil/ 90 SAE Gear Oil. You can think of it like reading a thermometer that lists the temperature in both Celsius and Fahrenheit.

Hopefully, this short guide explaining heavy-duty hydraulic oil grades helps you decide which fluid is best for your industrial application. If you still need help deciding, or need to find an equivalent oil classification, let Santi Oil Company help. We’re a heavy-duty hydraulic oil wholesaler dedicated to meeting all of your grease, oil, and lubricant needs.