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The Most Important Factors To Consider When Choosing A Gear Oil

Keep your equipment protected and find the gear oil best-suited for your applications 

The main purpose of gear oil is to protect the gears of your machinery. Today’s gearboxes are smaller and made of lighter weight material but are pushed to produce more power in increasingly harsh industrial environments. 

Gear oil plays a critical role in removing contaminants such as dirt, water, wear particles, and other foreign matter that can damage gears and bearings and impact efficient, smooth running of the gears. 

Even with regular lubricant maintenance, heat, higher loads and pressures, and contaminants such as water can compromise a gear system. The results can be expensive downtime, repair, or replacement costs. That’s why choosing the best gear oil for each application, while challenging, is important. 

Gear oil considerations 

When choosing the correct gear oil for each application, it best to follow the OEM’s recommended list while also taking into consideration: 

  • Viscosity – Often referred to as the most important property of a lubricating oil. 
  • Additives – The additives used in each gear oil will determine the lubricant’s general category and affect 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 the owner’s manual. Sometimes, however, the equipment operates outside the conditions for which the OEM’s recommendations were made. 

Constant gear rolling and sliding produces friction and heat. The heavier operating loads common in today’s industrial settings increase metal-to-metal contact or boundary lubrication, producing even more heat and pressure. On top of that, to meet longer drain intervals for environmental and cost reasons, the fluid stays in the system longer. Therefore, fluid cleanliness and performance retention are critical. 


High-viscosity gear oils generate heat from internal fluid friction and also may consume more power to turn the gears. The rate of oxidation in the fluid can increase, which decreases the fluid’s effectiveness and life. In addition, higher operating temperatures increase sludge and varnish formation, which can damage equipment by forming deposits that can block filters, oil passageways, and valves. 

On the other hand, lower viscosity gear oils generate less heat and, thereby, lessen the chance of exceeding recommended operating temperatures or damaging equipment. 

Gear oil viscosity is primarily chosen to provide a desired film thickness between interacting surfaces at a given speed and load. Because 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 for determining viscosity is the ANSI (American National Standards Institute) and AGMA (American Gear Manufacturers Association) standard ANSI/AGMA 9005-E02. 

Gear oil type and additives 

After selecting the viscosity grade, the basic type of lubricant must be chosen. The type of gear oil for any given application will be determined by the operating conditions. 

Gear oils are generally placed into three categories: 

R&O gear oil 

Rust and oxidation inhibited (R&O) gear oils do not contain anti-scuff additives or lubricity agents. R&O gear oils generally perform well in chemical stability, demulsibility, corrosion prevention, and foam suppression. These gear oils were designed for use in gears operating under high speeds, low loads, and with uniform loading (no shock loading). 

The Most Important Factors To Consider When Choosing A Gear Oil

R&O gear oils are the best choice for applications where all surface contacts operate under hydrodynamic or elastohydrodynamic lubrication conditions. They do not perform well or prevent wear under boundary lubrication conditions. 

Antiscuff (Extreme Pressure) gear oil 

Antiscuff or extreme pressure (EP) gear oils have some performance capabilities that exceed those for R&O oils. In addition to the properties listed for R&O gear oils, anti-scuff/EP gear oils contain special additives that enhance their film strength or load-carrying ability. 

The most common EP additive is sulfur phosphorous, which is a chemically active compound that alters the chemistry of machine surfaces to prevent adhesive wear under boundary lubrication conditions. 

Machine conditions that generally require anti-scuff/EP gear oils include heavy loads, slow speeds, and shock loading. 

In addition to sulfur phosphorous and zinc dialkyl dithiophosphate (ZDDP) anti-wear additives, several common solid materials are considered anti-scuff/EP additives, including molybdenum-disulfide (moly), graphite, and borates. 

One benefit of these additives is they do not depend on temperature to become active, unlike sulfur phosphorous compounds which do not become active until a high surface temperature is achieved. Another potentially negative aspect of sulfur phosphorous EP additives is they can be corrosive to machine surfaces, especially at high temperatures. 

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

Compounded gear oils 

The compounded oil is the third type of common gear oils. In general, a compounded gear oil is mixed with a synthetic fatty acid (sometimes referred to as fat) to increase its lubricity and film strength. The most common application for these gear lubricants is worm gear applications. 

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

Base oil selection 

High-quality mineral base oils perform well in most applications. In fact, mineral base oils typically have higher pressure-viscosity coefficients than common synthetics, allowing for greater film thickness at given operating viscosities. There are, however, situations where synthetic base oils are preferable. 

Many synthetic base oils have greater inherent resistance to oxidation and thermal degradation making them preferable for applications with high operating temperatures and, in some cases, allowing for extended service intervals. Additionally, synthetics perform better in machines subjected to low ambient temperatures due to their high viscosity index and low pour points. 

The high viscosity index also makes synthetic products suitable for a wider range of ambient temperatures, eliminating the need for seasonal oil changes. Some synthetics may also offer greater lubricity which reduces friction in sliding contacts. 

Phillips 66® Gear Oils 

Our gear oils provide advanced wear protection under heavy loads and in extreme temperatures—all while 
helping avoid downtime and premature wear to keep your equipment and jobs running smoothly. 

Learn more