What’s The Difference Between Gear Oil And Hydraulic Oil?

It’s true that money makes the world go around, but it’s oil that lubricates it. We use oil for just about every sort of industry today, from construction to pharmaceuticals, home appliances to travel, and everything in between. There simply is no one-size fits all oil type for all of this stuff despite it all coming from the same source, which is why you can’t use petroleum jelly in place of motor oil. That’d be like comparing a poodle to a grizzly bear because they’re both caniforms.

But that doesn’t apply to every type of oil. Sure, some of them are obviously extremely different, but then you have two with apparently similar functions on paper, like gear oil and hydraulic oil. They’re both lubricants and noncompressible fluids, so they’re interchangeable, right? Not at all, and the reason why is additives and chemical composition.

When you’re creating something for a specific purpose, you want to emphasize traits that are favorable for that purpose. If you know your oil needs to function in high-pressure environments, you use additives that work well for that. If you need to lubricate fast-moving gears, you want something different. The viscosities, protective qualities, tolerances, and operating temperatures of each fluid are vastly different from one another.

Think of it this way: No mechanic would ever recommend pouring brake fluid — no matter what type it is — into a manual transmission or gear oil into your brake system. But why, exactly? It’s because these fluids were designed to move and react to pressure in different ways. What are those ways though, and what’ll happen if you mess them up? Let’s discuss.

Gear oil goes by several different names depending on how it’s used, its viscosity, additives, and so on. You have manual transmission fluid (MTF), differential oil (simply diff oil), hypoid gear oil, synchromesh oil, helical, worm, bevel — all common names for specific types of gears or gearboxes. Put simply, different gears have different needs. Some of them build up a lot of friction and heat like hypoid gears, which is why it has its own unique hypoid gear oil typically used in differentials. Others spin really quickly, like those found in manual transmissions.

In general, gear oil is designed to act as a barrier between each set of teeth so that the metal itself isn’t in contact. Metal-on-metal friction creates wear and heat. Moreover, the fluid pulls that heat away from the working area, so it too gets hot, which is why transmission coolers are a thing. However, the gears in a manual transmission are generating a ton of sliding friction, so you need a fluid that’s sticky. Likewise, those gears place tremendous pressure upon the fluid itself. That leads us back to additives — different pressure ratings for gear oil have different designations, and thus different additives to fulfill those purposes.

All that is a long-winded way of saying that gear oil is an umbrella term, the same as hydraulic oil, and the two don’t mix — literally and figuratively. Hydraulic oil lacks the fundamental properties that gear oil possesses, and vice versa. Using hydraulic oil in a gearbox will break teeth faster than chomping down on a lump of granite.

The purpose of hydraulic oil is to act as a power transfer medium. You fill a hydraulic line with this fluid, apply pressure, and the incompressible fluid will transfer that pressure from Point A to B. There are numerous benefits to using a fluid as opposed to a solid arm — fluids are flexible, so you don’t need complicated joints; they’re cheap and easily produced, and there are lots of different types that specialize in different applications. Pro-tip: A motor oil substitute isn’t one of those applications.

That said, hydraulic oil’s main benefit is its incompressibility, but it also serves numerous other functions such as lubrication, heat transfer, anti-corrosion and oxidation, and so on. Much like gear oil, it’s about minimizing friction and protecting the working surfaces. However, hydraulic oil operates in a high-speed, high-pressure environment. It’s got a wide operating temperature range, it’s far less viscous than gear oil, and it’s extremely good at transferring energy.

Functionally speaking, gear oil in hydraulic lines is basically what would happen if you put in hydraulic oil that was incredibly thick and offered little of its secondary or tertiary benefits. So what happens if you use hydraulic oil that’s too viscous? You risk damaging the equipment because the pressure isn’t enough to move the fluid as efficiently, so you end up with sluggish performance and horrible lubrication.

It’s like a bicycle gearbox. If you try and pedal in first gear down a hill, your legs won’t be able to keep up — that’s hydraulic fluid in a transmission. If you try and take off uphill in top gear, that’s like using gear oil in hydraulic lines.