What You Need to Know When Selecting Gear Oil

What is Gear Oil?

Gear oil is a lubricant made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It is of high viscosity and usually contains organosulfur compounds.

Some modern automatic transmissions (integrated transmission and differential) do not use heavy oil at all, but instead, lubricate with the lower viscosity hydraulic fluid that is available under pressure in the automatic transmission. Gear oils make up about 20% of the lubricants market.

Most gearbox and differential lubricants contain high-pressure additives (EP) and anti-wear additives to deal with the sliding action of hypoid bevel gears. Typical additives include dithiocarbamate derivatives and sulfur-treated organic compounds (“sulfur-containing hydrocarbons”).

EP additives containing phosphorus/sulfur compounds are corrosive to yellow metals such as copper and/or brass used in bushings and synchronizers. The gear oils of the GL-1 class contain no EP additives and are therefore used in applications that contain parts made of yellow metals.

How do you know which Gear oil is the best fit for a given application?

It’s usually as simple as searching a service manual and selecting a product from the QPL (Qualified Product List). Unfortunately, this solution may not always provide optimal lubrication for a given gear set or maximum efficiency in managing lubricant inventory.

While some original equipment manufacturers (OEMs) provide general specifications that take into account relevant parameters, others only provide a general specification that may not even take operating temperatures into account.

It is therefore important that those responsible for selecting lubricants have a basic understanding of how lubricants are specified for the gearbox.

In addition to understanding and interpreting the device manufacturer’s specifications, it is important to understand why and be able to make changes if necessary.

There are numerous factors to consider when selecting industrial gear lubricants beyond simply selecting a product from the maintenance manual’s QPL, including product availability, operating conditions, preferred brand of lubricant, and product consolidation efforts. Correct selection of lubricant is a cornerstone of an excellent lubrication program.

A good understanding of this enables the lubrication technician to maximize the reliability of the machine under normal conditions and to use the lubricant specification as a problem solver under abnormal conditions.

Gear Oil Selection Criteria

In order to choose the best lubricant for a gear set, the following criteria must be addressed:

  • Viscosity. Often referred to as the most important property of a lubricating oil.
  • Additives. The additive package used in the lubricant will determine the lubricant’s general category and affects various key performance properties under operating conditions.
  • Base Oil Type. The type of base oil used should be determined by the operating conditions, gear type and other factors.


Choosing an appropriate viscosity grade is usually as simple as finding the recommendation in a component’s service manual. Unfortunately, the manual does not always exist or the machine operates outside of the conditions for which the OEM’s recommendations were made.

Therefore, it is important to understand the viscosity selection methods and the factors that affect the requirement.

The viscosity for a gear lubricant is chosen primarily to provide a desired film thickness between mating surfaces at a given speed and load. Since it is difficult to determine the load for most viscosity selection methods, the load is assumed and the determining factor becomes speed.

One of the most common methods of determining viscosity is the ANSI/AGMA 9005-E02 standard according to ANSI (American National Standards Institute) and AGMA (American Gear Manufacturers Association). In this method, assumptions are made with regard to the load, the viscosity index and the pressure-viscosity coefficient of the lubricant.

The table in Figure 1 applies to spur, helical and chamfered closed gear sets. Other diagrams exist for worm gears and open gears. In order to use this method, the type of gear set, the gear geometry, the operating temperature, and the speed of the slow gear must be determined.

Gear Oil Viscosity grade

After calculating the pitch line speed of the slowest gear in the unit, the required viscosity grade can be read from the table using the most likely operating temperature of the unit.

It is important to note that this method assumes the viscosity-temperature relationship of the lubricant (viscosity index = 90). If the VI of the lubricant differs from this value, additional tables are included for oils with VI = 120 and 160, or a viscosity-temperature diagram can be used to interpolate the appropriate ISO viscosity grade.

Gear Lubricant Type and Additive Selection

After selecting the viscosity class, the basic lubricant type must be selected. Although there are many variations, gear lubricants can generally be divided into three categories: R&O, Anti scuff, and Compound. The type of gear lubricant that best suits a particular application is determined by operating conditions.

Since there are no standard guidelines to support this determination, the choice is somewhat subjective. Many device manufacturers set a viscosity requirement and leave this decision to the end user.

Others will choose to be conservative and specify EP lubricants for the applications. It is therefore important to understand the general conditions that affect this requirement.

R&O Gear Lubricants

Rust and oxidation inhibiting (R&O) gear lubricants do not contain anti-fuff additives or lubricants. R&O gear oils generally perform well in the categories of chemical stability, demulsibility, corrosion protection and foam suppression. These products are designed for use in gearboxes that operate at relatively high speeds, low loads, and uniform loads (no shock loads).

These lubricants are the best choice for applications where all surface contacts operate under hydrodynamic or elastohydrodynamic lubrication conditions. They do not work well or prevent wear under boundary lubrication conditions.

Antiscuff (Extreme Pressure) Gear Oil

Antiscuff gear lubricants, commonly referred to as Extreme Pressure (EP) lubricants, have some performance characteristics that are superior to R&O oils. In addition to the properties listed for R&O lubricants, Antiscuff lubricants contain special additives that improve their film strength or load-bearing capacity.

The most common EP additives are sulfur phosphorus, chemically active compounds that change the chemistry of machine surfaces in order to prevent the adhesive from wearing out under boundary lubrication conditions.

In less severe applications, anti-wear additives can also be used to provide wear protection under boundary lubrication conditions. Machine conditions that generally require anti-shoe lubricants include heavy loads, slow speeds, and shock loads.

In addition to the anti-wear additives sulfur phosphorus and zinc dialkyldithiophosphate (ZDDP), several common solid materials are considered antifuff additives, including molybdenum disulfide (moly), graphite, and borates.

An advantage of these additives is that they do not depend on the temperature in order to become active, in contrast to sulfur-phosphorus compounds, which only become active when a high surface temperature is reached. Another potentially negative aspect of phosphorus sulfur EP additives is that they can attack machine surfaces, especially at high temperatures.

This type of additive can also be corrosive to yellow metals and should not be used in applications with components made from these materials such as worm gears.

Compounded Gear Oil

The compound gear lubricant is the third type of common lubricant. Generally, a compound lubricant is mixed with a synthetic fatty acid (sometimes referred to as a fat) to increase its lubricity and film strength. The most common uses for these gear lubricants are worm gear applications.

Because of the sliding contact and adverse effects of EP agents, composite lubricants are generally the best choice for these applications. Compounded oils are also known as cylinder oils because these lubricants were originally formulated for steam cylinder applications.

Base Oil Selection

High-quality mineral base oils are well suited for most applications. In fact, mineral base oils typically have higher pressure-viscosity coefficients than common synthetics, which allows for greater film thickness for given operating viscosities. However, there are situations when synthetic base oils are preferable.

Many synthetic base materials have greater inherent resistance to oxidation and thermal degradation, making them preferred for high operating temperature applications and, in some cases, allowing longer service intervals.

In addition, synthetic perform better in machines exposed to low ambient temperatures due to their high viscosity index and low pour points.

The high viscosity index makes synthetic products suitable for a wider range of ambient temperatures so that no seasonal oil changes are required. Some synthetics may also offer greater lubricity, which reduces friction on sliding contacts.

The selection of lubricants for industrial gears is similar for most applications. There is no particular property or value to make a good specification. Determining the best choice for a particular application requires selecting the right viscosity, base oil, type of lubricant and evaluating the appropriate performance characteristics.


What is gear oil used for?

Gear oil is a lubricant designed for use in transmissions, manual gearboxes, differentials, transaxles, and transfer cases in your car or truck. Gear oil helps your transmission run smoothly. More importantly, it helps protect critical internal components in your car’s gear systems from wear and heat damage.

Is gear oil and engine oil same?

Gear oil and motor oil are not the same things, and they are not interchangeable. Gear oil is specially formulated for use on gears. Motor oil You guessed it – it’s for motors.

Which oil is best for the gearbox?

Some gearbox manufacturers specify a “pure” or “straight” mineral oil gear lubricant. This means it should not contain chemically active EP additives, although it may still contain various other highly desirable additives.

What is a typical gear oil?

GL-4 – The world’s most common base oil grade, the oils of which contain good volumes of extreme pressure additives. GL-5 – Containing many more additives than GL-4 oils, GL-5s are used to create gear oils with extreme load resistance, protecting systems such as hypoid gears.

Is the gearbox same as the transmission?

The difference between gearbox and transmission is that, while these two are two separated units, the transmission refers to the whole drivetrain which includes the gearbox, clutch, prop shaft, final drive shafts, and differential.

What is 80w90 gear oil used for?

The oil known as 80W-90 GL-5 is a lubricant for the gears in your vehicle. It is used in automatic and manual transmissions and assists the gears in shifting smoothly.

Can I use gearbox oil in my engine?

Gearbox oil has a higher viscosity than engine oil and this is because it better protects the gears, ensuring you have a smooth transition between them. The higher viscosity is also essential due to the fact there is no pump to push the oil around the gearbox. Therefore, you cannot use engine oil in your gearbox.

How can you tell oil from gear oil?

gear oil, you may be tempted to think they’re the same thing, but they’re not. Gear oil is specifically designed to protect, lubricate, and cool gearing systems. Engine oil lubricates the bearings and protects your car’s engine from the additives in gasoline.

How many km should I change gear oil?

Manual gearboxes usually require a gearbox oil change every 30,000 to 50,000 mi (48,000 to 80,000 km). Automatic gearboxes usually have gear oil change intervals of between 60,000 to 100,000 mi (96,000 to 160,000 km). It’s advisable that you change your gearbox oil filter at the same time as your oil.

What grease is good for gears?

The best grease for plastic gears is generally white high-temperature grease or high-viscosity silicone grease.

What is the viscosity of gear oil?

The main thing you’ll notice about gear oil is its viscosity. It’s much thicker than motor oil, with an average motor oil being 5W/30 and the average gear oil being 75/90 (see this article for an depth explanation of oil viscosity index).

What does gear oil look like?

Both the HD and Regular Gear Oil are a yellow or brownish color. A rule of thumb for the models that require the HD Gear Oil is the oil filter. If the engine uses an N26-13440-02-00 oil filter, or an N26 for short. Then that engine requires the HD Gear Oil, which is to the left and is also a brownish or yellowish color.

What is GL 3 gear oil?

Champion gl-3 all-purpose gear lube is a high quality, moderate duty, extreme pressure, all-purpose gear lube for use in automotive, agricultural and industrial applications. For use where the manufacturer specifies AGMA EP, AGMA 9005-D94, US STEEL 224, Cincinnati Milacron, Extreme Pressure, or GL-3 gear lube.

What Color is gear oil?

Gearbox specialists agree that the normal color of the liquid in the automatic gearbox should be pink. So, if your transmission fluid is brown or black, then your gearbox has been compromised.

What is the difference between transmission oil and gear oil?

The simplest answer to the difference between transmission fluid and gear oil is in the purpose for which they are designed. Transmission fluid is engineered to work with automatic transmissions, whereas gear oil is typically intended to work with manual-type gearboxes.

Is Thicker gear oil better?

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 loads.

Where do I put gear oil?

The oil should be level with the bottom of the plughole. To remove the plug, reach through the engine bay, or jack up the car, support it level on axle stands at front and rear, and lie underneath the gearbox. More rarely, you can reach the plug through an inspection hole in the car floor beneath the carpet.

What happens when gearbox oil is low?

A healthy transmission will shift between gears smoothly and there won’t be any slippage. Low fluid levels can cause gears to miss, which will result in a grinding sensation.

How many Liters of oil does the gearbox take?

Cheers. About two quarts or liters. Just fill it up until the fluid comes back out of the opening, then put the plugin back in it. It’s exactly the amount the manufacturer specifies for your specific gearbox.

How do you know when to change gear oil?

If the gear oil appears dark or gritty, then it must be changed. If the gear oil level is low, top it up. If topping up the transmission oil doesn’t help with this, you may have internal components that have failed. We recommend booking an inspection with your local mechanic to find the issue.

What is worm gear oil?

The types of oils most commonly used to lubricate worm gears are compounded mineral oils, EP mineral gear oils, and synthetics. Each has its own unique characteristics and all three types are used successfully.

Why is my gear oil black?

If your transmission fluid is dark brown or black, it needs to be flushed and changed right away. The reason why transmission fluid changes from bright red to brown to black as it ages is that is it oxidizing. Oxidation is bad for transmission fluid.