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Where is gear oil used?

While the primary application of gear oil is in transmissions for automobiles, these oils are made to be used pretty much anywhere a gearbox can be found.

This naturally means that they need a diverse range of properties, since the varied materials and conditions of gearboxes can vary quite a bit. It can be hard to pinpoint which oil is the best for any individual application, especially due to the varying environments in which they’re used.

Gears produce quite a bit of heat and friction as they work against each other, which means that an oil is pretty much necessary to avoid abrasive damage between the gears which will lead to the eventual degradation of a system.

The majority of gear oils on the market are geared towards automotive purposes, particularly for manual transmissions. These are usually termed as “universal” gear oils and while they can be used for industrial processes they often contain additives which can be detrimental to machinery.

Dedicated gear oils on the other hand, are tailored specifically for industrial processes and contain a different set of additives and protectant compounds.

Oddly enough, gear oils are one situation where using full synthetic oils isn’t always desirable. Base mineral oils often perform better than synthetics at lower temperatures, having a greater film strength by virtue of a higher pressure-viscosity coefficient.

On the other hand, synthetic oils are desirable in high temperature applications due to their increased ability to resist oxidation and thermal degradation. They can also handle a wider range of ambient temperatures while maintaining their viscosity.

Gearbox Oil

Gear oils have a base grade which determines most of their qualities. Most of them are obsolete for automotive purposes at this point, with only GL-4 gear oil and GL-5 gear oil being used in modern vehicles.

They’re all a little bit different.

GL-1 gear oil

These oils are designed primarily for low-use manual transmissions. They usually operate under such low pressure conditions that there are only minimal additives needed. Friction modifiers and extreme pressure additives disqualify an oil from performing in this category.

GL-2 gear oil

These gear oils are used for worm-gear type axles, and can have additional additives on top of what is allowed in GL-1 oils. They’re still not allowed to use extreme pressure additives.

GL-3 gear oil

These oils are used for manual transmissions where the qualities of a GL-1 or GL-2 oil won’t suffice, but aren’t quite requiring the amount of load that a GL-4 oil can handle. They’re usually used for manual transmissions which are operating under moderate to severe conditions. They have a light extreme pressure effect.

GL-4 gear oil

These oils possess a moderate level of extreme pressure additives and are the most commonly used gear oil in the modern world. The majority of transmissions use this kind of oil.

GL-5 gear oil

These oils possess a very high level of extreme pressure additives. They’re primarily used with hypoid gears and other extremely loaded systems.


Gear Oil

Like any other specialty oil, gear oils have their own grading system. For automotive purposes it’s important to remember that you can’t just dump motor oil in the transmission. While motor oils are formulated for minimal interactions with gasoline, transmission oils have a much different set of requirements.

There are two separate grading systems for gear oils.

For universal, or automotive, gear oils there are two sets of standards. Those which are destined for use in transmissions are graded according to SAE standards. You might, for instance, see 80W-90 gear oil or 75W-140 gear oils on the shelves.

Similar to motor oils, the numbers are pretty easy to read. Essentially, the first number of a multigrade oil, preceding the W, is the performance at 0°C while the second number rates it’s performance at 100°C.

So a 75W-140 gear oil has similar performance to an SAE 75 gear oil while at 0°C. The single grade oils performance would only be measured at 100°C. Gear oils can get remarkably close to performing almost uniformly across the spectrum, with grading like 75W-85 gear oil being fairly common.

This doesn’t mean much to most end consumers, who’ll simply want to match off with their owner’s manual for the vehicle.

Things aren’t quite as complicated when looking at industrial gear oils, however. They’re most often rated with a single grade but it’s on a different curve than those destined for automotive use.

ISO grades are internationally agreed upon standards for oils used in industry. ISO 68, for instance performs at 67.5cSt at 40°C. ISO grades tend to be much easier to read, since they’re tied to the centistokes of viscosity of a substance at 40°C. They’re not an exact measurement but they’re close.

Viscosity and gears

Unlike many oil applications, where slippery tends to be better, gears have different requirements depending on the system in question.

As a general rule high viscosity oils are best for low speed, loaded gears with a rough surface. Higher viscosity provides a thicker film, higher wear resistance, and less deformation of the gears as time goes on.

Low viscosity oils, on the other hand, are best for high speed systems with lower load. They provide a thinner film and better cooling to match the higher speed of the gearbox in question.

As always, it’s usually best to just consult a user manual to make sure you’re on the right track when you’re picking out your oil.

Other desirable qualities in gear oils

Just like any other specialized oil, gear oil has some specific requirements outside of being slippery and being able to handle heat and friction.

This is true for both industrial purposes and for automotive transmissions. Gear drives come in a wide variety of designs, however, which means that you’ll need to match the oil you’re using to the particular type of machinery being used.


In open gear designs, adhesion is required. By holding to the metal of the gears, it provides a thin foam which helps to prevent metal-on-metal contact.

This quality can be negative in most other designs, however, since it tends to induce higher friction and isn’t very cooling.

Low emulsion formation


Gear oils, more than almost any other commonly used oil, need to emulsify water in order for it to be removed from the gears. This means that oils which easily emulsify with water, as opposed to separating from it, are highly undesirable.

This quality is usually maintained through the use of additives. By separating from water, instead of allowing the liquid to emulsify within it, gear oils can rapidly address the problem to keep corrosion and oil oxidation to a minimum.

This is much more important for industrial applications than in automotive ones for obvious reasons.

Foam inhibition

Because of the way that gear drives work, serious foam formation is a very real probability while things are in motion. This means that foam needs to be inhibited through the use of antifoaming agents in order to keep things from getting out of hand.

While not as disastrous as in hydraulic operations, foam can still cause a hindrance in gear systems.

Minimal interactions with components

While most mineral oils have little to no interaction with ferrous metals, there are a number of parts in any given gearbox which may interact with oils without the right additives. These include rubber gaskets and seals, and a good gear oil should be formulated to minimize interaction with these as well.

Thermal stability

Gear oils need to remain stable under the heat generated by the gearbox in question. While all gears are going to produce some heat, the requirements for heat resistance vary quite a bit depending on the application.

Faster gears tend to heat up more than slower ones, but slower gears are often under higher loads which leads to more pressure and heat so remaining stable is extremely important.

An oil which isn’t stable will degrade faster and create more sludge within a system in a shorter amount of time than one with a higher thermal stability. There are oils available for even the most extreme situations, so proper choice of lubricants in machinery is just a matter of finding the right oil for the system in question.

Types of gear oil

Like most types of specialized oils, gear oils come with a wide variety of different additives which are designed to handle different situations. In many cases, there are also drawbacks to the additives, which means that making sure the correct additive package is used in the right situation is of paramount importance.

R&O gear oil

R&O stands for rust and oxidation. These oil packages are designed to reduce the amount of corrosion within the gearbox they’re used in. They fall within ISO grades 32-320.

A majority of gear oils will have some level of protection from rust and oxidation, it’s one of the best ways to protect the gearbox and extend the life of an oil.

Rust protection keeps the gears and other pieces of machinery from corroding as the oil goes through the system and also provide some protection from moisture which can make its way into the machinery.

Oxidation protection keeps the microscopic debris which inevitably separates from the gears from oxidizing within the oil, prolonging the useful lifespan of the gear oil.

Extreme pressure gear oils

Extreme pressure gear oils are designed to withstand heat and pressure to a much higher degree than any other type. This naturally means a lot of additives, and it can prove to be hazardous if you’re not careful about what kind of machine they’re put in.

Most of these agents are made with a base of chlorine or sulfur, which can prove hazardous for some metals. For instance the bronze gears often used in worm-drives can have their surface softened by exposure.

This means they should really only be used when called for by a manufacturer. Other antiwear additives, such as zinc-based protectants, are desirable for applications where metals easily effected by chlorine or sulfur are present.

Compound gear lubricants

By combining a gear oil with fatty oils, the viscosity of a gear oil can be raised to quite high levels. When combined with the right additives, you end up with an oil which is suitable for low speed, high-pressure applications while still providing high lubricity.

Synthetic gear oils

Unlike motor oil, where synthetic oils are highly desirable, synthetic gear oils are more of a niche product. Created from polyalphaolefin and additives, these oils are primarily used for extreme machine conditions.

Whether it’s super low or super high temperatures, synthetic gear oils are the best choice when you’re looking at conditions which other oils simply won’t function in. They’re created in a huge array of viscosity, with gear oils available in ISO grades 32 to 6800 being produced.

They’re also useful for situations where the pressures are high.


Gear oils are a complex subject, but their varied uses are one of the driving forces of modern industry and infrastructure. At least a basic understanding of them should be on everyone’s agenda.

Thankfully, the meanings behind the different grades and such aren’t very opaque, instead it’s a relatively simple process to figure out which is the best for most purposes. So whether you’re working with a new machine or trying to find the right oil for your transmission, a bit of research will give you a leg up.

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