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.