What's The Difference Between Hydraulic Fluid And Hydraulic Oil?

Hydraulic fluid is the lifeblood of the world's machinery. You'll see it in everything with moving parts that uses the principle of compression and fluid transfer — from construction equipment to your car's transmission, it all relies upon hydraulic fluid.

It's not a complicated concept to understand, either; all you're doing is transferring energy. Imagine you're at Point A and want to move something at Point B. An efficient way to do that is to make a pipe and fill it with water. Since water can't compress, if you were to push down, the water will transfer that energy into the thing on the other end of the pipe (as well as you, that's Newtonian physics in action). Presto, you've just created a hydraulic line.

A practical application of this is your car's brakes. When you press on the brake pedal, you're basically pushing a piston that then applies pressure to the brake lines. Okay, the braking system is more complex, with a brake booster, anti-lock systems, and so on. But all of that acts upon your brake fluid, which is a hydraulic fluid.

Hydraulic oil, on the other hand, is just another subtype of hydraulic fluid. It's like how your engine oil is a subtype of refined oil in general. Two different kinds of hydraulic fluid may be as closely related as engine oil is to, say, petroleum jelly. In the case of hydraulic oil versus fluid, hydraulic oil has specific properties that differentiate it from other subtypes in a similar way to that comparison. Let's explore exactly what hydraulic oil is and how it relates to the hydraulic fluid family tree.

Before we can break down hydraulic oil, let's first understand the basics of hydraulic fluid, because it encompasses a broad range. Automatic transmission fluid is a type of hydraulic fluid, as is the fluid which powers the pistons of excavators. Because of its more generalized applications, hydraulic fluid is found basically anywhere with powered tools; it's most prominent in industrial and construction equipment, where you can visibly see the high-pressure lines.

The main property shared between all of these is they can't be compressed. This makes hydraulic fluid excellent as a power transfer medium — you apply pressure to one part of the system, and that pressure is transferred to another part. Another use is to transmit power between two bodies like a fluid coupling, which is what automatic transmission fluid does. You're using your car's torque converter to send power from the flex plate to the gearbox, which is done with transmission fluid spinning around like a clutch made out of water.

Hydraulic fluid does far more than just transmit force, though, otherwise we'd just use water for everything. It also provides lubrication for the working parts, preventing corrosion and heat buildup. It helps cool systems by spreading out the heat generated from the working surfaces. It carries contamination away from vital components inside the machinery. It's more of an all-purpose fluid that is available in a wide variety of viscosities and chemical compositions, depending on the intended application. One of those compositions, certainly among the most popular, is hydraulic oil.

In short, hydraulic oil is just the most common type of hydraulic fluid, so much so that people often use the term interchangeably. But what is it, specifically? Well, as the name implies, hydraulic oil is petroleum-based, not unlike the engine oil in your car. In fact, your car's engine oil and some hydraulic oil are both fairly similar on paper, with the same base sources and general properties. The main difference between these two oil types is in the additives, which determines what they're used for.

The main difference is flammability. Hydraulic oil is explosive, which isn't exactly ideal in a high-heat environment like a car engine. Similarly, it doesn't like high-pressure environments, either, since spraying hydraulic fluid that's been heated and forced to move quickly can lead to ignition. That's why specialized hydraulic fluid was developed in the first place — unlike hydraulic oil, non-petroleum based fluids won't ignite the same way under pressure, and have the additives necessary to keep that machinery running (whereas engine oil does not).

Nevertheless, hydraulic oil remains the most popular general-use hydraulic fluid. It's extremely versatile, with numerous blends for different applications — companies produce lines in conventional, synthetic, and blend, plus additional bonuses like wear resistance or flame-retardant properties. Like hydraulic fluid, it depends on the formula and what you're using it for, but it all ultimately falls under the "hydraulic oil" umbrella — which then falls under the "hydraulic fluid" umbrella above it. Basically, all hydraulic oil is also hydraulic fluid, but not all hydraulic fluid is oil.