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How to Read a Gear Oil Viscosity Chart

How to Read a Gear Oil Viscosity Chart

Save this chart for your own use. As an AMSOIL dealer I  use it several times weekly to show customers how things like a 20W-50 motor oil (motorcycle oil) can be also used as a gear lube.. Also how ranges of one oil is significant as a 90WT for differentials..

This comparative viscosity chart can help determine if two or more lubricants have similar viscosities.

by Joel Youngman|January 6, 2024

Viscosity, defined as a fluid’s resistance to flow, is one of the most important characteristics of a lubricant. Some of the informal terms used to describe the viscosity of a relatively free-flowing fluid, such as water, include thin, light and low. Terms such as thick, heavy or high suggest a fluid with strong resistance to flow, such as honey. However, these terms are general and difficult to measure.

More specific classifications give us a better idea of how fluids move, but you’ve likely seen at least a few different ways to designate viscosity:

Society of Automotive Engineers (SAE) grades for automotive motor oils (e.g. SAE 5W-30)

Society of Automotive Engineers (SAE) grades for automotive gear oils (e.g. SAE 75W-90)

American Gear Manufacturers Association (AGMA) grades for industrial gear lubricants (e.g. AGMA 5)

International Standards Organization (ISO) grades for hydraulic fluids and industrial gear lubricants. Established to represent a universally accepted grading system (e.g. ISO 100)

Comparative Gear Oil Viscosity Chart

Considering there are multiple standards (that use different scales) for designating viscosity, a comparative viscosity chart can help determine if two or more lubricants have similar viscosities. But how do you read a gear oil viscosity chart?

Just read it horizontally. For example, an SAE 60 motor oil has a similar viscosity to an SAE 90 gear oil, an AGMA 6 gear lubricant and an ISO 320 hydraulic fluid/gear lubricant. The corresponding kinematic viscosity and Saybolt viscosity are also referenced on the chart.

Kinematic Viscosity

Commonly seen on a lubricant’s data sheet, kinematic viscosity describes a fluid’s visible tendency to flow. Think of this as the time it takes to watch a fluid pour out of a container.

This tendency to flow is expressed in units suggesting the volume of flow over time, called centistokes (cSt). Kinematic viscosity is usually tested at both 40°C and 100°C.

Saybolt Viscosity

Although centistokes are the most common unit of measurement when determining kinematic viscosity, results may also be reported in Saybolt Universal Seconds (SUS). Viscosity reported in SUS is becoming increasingly rare, but you may still come across it when reading lubricant product information. Saybolt viscosity is usually tested at both 100°F and 210°F.

How Often Should I Change Differential Fluid?

How Often Should I Change Differential Fluid?

Fluid change intervals depend on your vehicle, driving conditions and gear oil quality.

_by David Paiuilldorf | july 26, 2023

A differential is a set of gears that allows a vehicle’s driven wheels to revolve at different speeds when going around corners or over rough terrain. Those gears require lubrication, which is commonly called differential fluid, gear oil, or gear lube.

How often you should change differential fluid depends on your vehicle, driving conditions and fluid quality. If you drive primarily on the highway in temperate conditions and rarely tow or haul, you won’t need to change the differential fluid very often.

However, severe operating conditions break down differential fluid more quickly and place greater stress on the gears and bearings, inviting wear that reduces their lifespan.

Greater load capacity, less lubrication.

Truck manufacturer’s ongoing arms race for the highest towing capacity has resulted in trucks that place more stress on differentials than their predecessors. Meanwhile, differential fluid capacities have largely decreased or remained the same.

For example, compare a 1996 Ford F-250* Crew Cab to the 2021 version. Back in 1996, maximum towing capacity was 10,500 lbs. using a rear differential that held 3.75 quarts of gear oil. The 2021 model offers up to 15,200-lb. towing capacity using a rear differential that holds between 3.3 and 3.5 quarts of fluid.

This means less lubrication is responsible for protecting gears among higher heat and stress. In this environment, inferior lubricants can shear and permanently lose viscosity. Once sheared, the fluid film weakens, ruptures and allows metal-to-metal contact, eventually causing gear and bearing failure. That’s not good!

Increased temperatures in differential

As temperatures rise, lubricants lose viscosity and the extreme pressure between gears can shear the lubricant film, causing increased metal-to-metal contact, friction and heat. Friction and heat create a vicious cycle known as thermal runaway that spirals upward, leading to increased wear and potential gear failure.

Oh and let me tell you a quick story about AMSOIL diff fluid. Under thermal runaway the heat causes the gears or metal to expand just enough to act like a brake. We’ve had RVs comment on reaching 5 to 7 more miles per hour on mountain passes due to the reduction of heat using the AMSOIL Severe Gear. See what I’m saying?

Severe Service Differential Fluid Solution

If you drive via Severe Service towing conditions or haul heavy loads frequently, especially in warmer temperatures, most manufacturers recommend changing the differential fluid more often, sometimes as often as every 30,000 miles. This applies to most work trucks and anyone who pulls a camper, boat or trailer regularly

The best way to determine the recommended mileage interval for differential fluid changes is to check your owner’s manual. Use a high-quality synthetic gear oil to reduce gear wear and maximize your vehicle’s ability to tow and haul.

Amsoil’s Severe Gear Squeeze or Easy-Pack

Amsoil Squeeze Packs cut the differential oil change process in half!

AMSOIL SEVERE GEAR Synthetic Gear Lube is specifically designed for severe service. It maintains viscosity better than other gear lubes under rigorous use and contains advanced anti-wear additives for additional protection.

SEVERE GEAR often costs less than OEM-branded gear oil, and the AMSOIL Easy-Pack or I call Squeeze Pack eliminates the frustrations of changing gear oil. Its flexible design lets you easily reach inconvenient fill holes and squeeze all the gear lube out of the package. Less mess, less waste, less frustration.

And hey, by the way most auto parts stores don’t even carry a known name brand GL5 gear lube these days. Tell your auto parts store to PLEASE carry AMSOIL as it’s about the only real performance player out there and the only one that really works..

 

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Gear Oil vs. Engine Oil: What’s The Difference?

Gear Oil vs. Engine Oil: What’s The Difference?

Gear oil uses a different viscosity classification and different additives.

by Joel Youngman|May 9, 2022

High-quality gear oil must lubricate, cool and protect geared systems while carrying damaging wear debris away from contact zones and muffling the sound of gear operation. In this post, we’ll look at the differences between gear oil vs. engine oil.

Differentials, manual transmissions and industrial machinery gears often require protection under extreme temperatures and pressures to prevent wear, scuffing and other damage that results in equipment failure. Protection against oxidation, thermal degradation, rust, copper corrosion and foam is also important.

Gear Oil vs. Engine Oil: Viscosity Comparison

Gear oil differs from engine oil. While many motorists may assume SAE 90 gear oil is thicker than SAE 40 or 50 engine oil, their viscosities are the same, as this gear oil viscosity chart shows.

 

One major difference between the two are in the additives they use.

Motor oil contains additives such as detergents and dispersants to combat byproducts from gasoline or diesel ignition. Because an internal combustion engine has an oil pump and lubricates the bearings with a hydrodynamic film, extreme-pressure additives such as those used in gear oils are not necessary.

Gear Oil Undergoes Boundary Lubrication

Engine oils and gear oils both have anti-wear additives, and they both must lubricate, cool and protect components. Gear oils, however, may be placed under extreme amounts of pressure, creating a propensity for boundary lubrication, a condition in which a full-fluid lubricating film is not present between two rubbing surfaces.

For example, differentials in cars and trucks have a ring-and-pinion hypoid gear set. A hypoid gear set can experience boundary lubrication, pressures and sliding action that can wipe most of the lubricant off the gears. Extreme-pressure additives are used in gear oil to combat this extreme environment.

Gear Oil vs. Engine Oil: Additional Differences

Rust & Corrosion Protection

Because many of the components found in the drivetrain consist of ferrous material, gear oil must prevent rust and corrosion to other materials. Rust and corrosion problems are not nearly as prevalent in engines.

Shock-Loading

The many small and intricate components that make up gear sets can be quite noisy and may be subjected to shock-loading, which occurs when components are quickly placed under intense load, like when accelerating with a powerful engine.

The viscosity and extreme-pressure formulation of gear oil quiets gears and dissipates shock-loading.

Foam

The rotating motion of gears also tends to churn the lubricant, leading to foam. If a gear oil foams, its load-carrying capacity is significantly reduced because the air suspended within the oil is compressible.

For example, when the gear teeth contact, any trapped air bubbles compress, reducing the thickness of the separating oil film. In turn, this reduction could lead to direct metal-to-metal contact between gear teeth and result in accelerated wear.

Typical Drivetrain Fluid Additives

Much like with motor oil, the additives included in gear oil either enhance existing properties or impart new ones. Drivetrain-fluid additives include the following:

  • Extreme-pressure and anti-wear agents minimize component wear in boundary lubrication situations.
  • Pour-point depressants improve low-temperature performance.
  • Rust and corrosion inhibitors protect internal components.
  • Oxidation inhibitors reduce the deteriorating effects of heat, increasing the oil’s service life.
  • Viscosity index improvers allow a lubricant to operate over a broader temperature range.
  • Anti-foam agents suppress foam and dissipate entrapped air.
  • Friction modifiers – The required degree of friction-reduction can vary significantly between different pieces of equipment in drivetrain applications. In some cases, friction modifiers may be required to obtain the desired results.

Common Gear Designs

Gear designs vary depending on the requirements for rotation speed, degree of gear reduction and torque-loading. Transmissions commonly use spur gears, while hypoid-gear designs are usually used as the main gearing in differentials. Common gear types include the following:

Spur Gears

Spur (straight-cut) gears are widely used in parallel-shaft applications, such as transmissions, due to their low cost and high efficiency. The design allows the entire gear tooth to make contact with the tooth face at the same instant. As a result, this type of gearing is subjected to high shock-loading and uneven motion. Design limitations include excessive noise and a significant amount of backlash during high-speed operation.

Bevel Gears

Bevel (straight- and spiral-cut) gears transmit motion between shafts that are at an angle to each other.

Primarily found in industrial equipment, as well as some automotive applications (differentials), they offer efficient operation and are easy to manufacture.

As with spur gears, they are limited due to their noisy operation at high speeds and are not the top choice where load-carrying capacity is required.

Worm Gears

Worm gear sets employ a specially machined “worm” that conforms to the arc of the driven gear. This design increases torque throughput, improves accuracy and extends operating life.

Primarily used to transmit power through nonintersecting shafts, this style of gear is frequently found in gear-reduction boxes as it offers quiet operation and high ratios. Its downfall is its low efficiency.

Hypoid Gears

Hypoid gear sets are a form of bevel gear, but offer improved efficiency and higher ratios over traditional straight-bevel gears. Commonly found in axle differentials, hypoid gears are used to transmit power from the driveline to the axle shafts.

Planetary Gears

Planetary gear sets, such as those found in automatic transmissions, provide the different gear ratios needed to propel a vehicle in the desired direction at the correct speed.

Gear teeth remain in constant mesh, which allows gear changes to be made without engaging or disengaging the gears, as is required in a manual transmission.

Instead, clutches and bands are used to either hold or release different members of the gear set to get the proper direction of rotation and gear ratio.

Helical Gears

Helical gears differ from spur gears in that their teeth are not parallel to the shaft axis; they are cut in a helix or angle around the gear axis. During rotation, parts of several teeth may be in mesh at the same time, reducing some of the loading characteristics of the standard spur gear.

However, this style of gearing can produce thrust forces parallel to the axis of the gear shaft. To minimize the effects, two helical gears with teeth opposite each other are used, which helps cancel the thrust during operation.

Herringbone Gears

Herringbone gears are an improvement over the double helical gear design. Both right- and left-hand cuts are used on the same gear blank, canceling out any thrust forces. Herringbone gears are capable of transmitting large amounts of horsepower and are frequently used in power transmission systems.

Gear Design Dictates Gear Oil Design

Differences in gear design create the need for significantly different lubricant formulations.

For instance, hypoid gears normally found in automotive differentials require GL-5 concentration and the performance of extreme-pressure additives due to their spiral sliding action.

Most manual transmissions have helical gears that do not require GL-5 performance. The helical gear is almost a straight-cut gear, but on an angle. There is spiral action and very little sliding action, and there is less need for extreme-pressure additives. GL-4 gear lubes provide less extreme-pressure additives than GL-5 lubes.

AMSOIL Gear Oil Recommendations

When comparing gear oil vs. engine oil, there are a number of differences between formulations. That’s why it is important to always use the correct oil for the correct application.

AMSOIL offers premium synthetic drivetrain lubricants to meet the needs of nearly every application. Check out our Product Guide to find what you need

Watch Scott Judnick Go Big – And Why We’re Into Snow

Watch Scott Judnick Go Big – And Why We’re Into Snow

At AMSOIL, we like to do things big. And when it comes to the world of Snocross, Scott Judnick of Judnick Motorsports likes to do things just as big. Check out his story below.

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It’s About the People

Twenty-two years ago, Scott Judnick took his sons racing. Within just a few years he was running a rig across the country to race. His two sons developed into professional riders complete with mechanics and trailers set-up for the AMSOIL Championship Snocross Series. Find Judnick anywhere on the track or in the pits and you are sure to be greeted with a smile and a “How are ya?”

For Judnick, it’s all about the people within the racing community. Fielding the dreams of the three young racers on his team is just a bonus.

Overcoming Adversity

Judnick went into the 2018-19 Snocross season with notable riders expected to dominate the Pro, Pro Lite and Sport classes. The season started on a high note with Sport rider Carson Alread taking the checkers to open the season in Duluth, Minn.

Noticeably absent from the Friday night DOMINATOR race was Pro Lite rider Nick Lorenz. A re-aggravated knee injury forced Lorenz to take it easy opening weekend. After further observation, he underwent surgery that ended his season before it started. Making matters worse for the team, a scary landing during practice in Canterbury, Minn., left Alread sidelined for the remainder of the season, too.

But that didn’t stop Judnick from continuing to compete. He signed Canadian standout RJ Roy, along with Pro rider Corin Todd. Roy has proven he can hang with the big boys, landing just short of the podium multiple rounds.

AMSOIL Products Keep Sleds Running Strong

Race sleds operate in extreme conditions. Judnick relies on AMSOIL DOMINATOR Synthetic 2-Stroke Racing Oil to keep his race sleds running in those extreme conditions.

“Our engines are tuned to run on the very edge and placed under extreme demands in extreme weather conditions,” said Judnick. “We’ve been using DOMINATOR since its inception and it has never let us down.”

DOMINATOR® Synthetic 2-Stroke Racing Oil

Buy DOMINATOR Synthetic 2-Stroke Racing Oil

Not to be forgotten, the chaincases on these sleds also need attention. Judnick uses AMSOIL Synthetic Chaincase & Gear Oil to protect his sleds’ chaincases.

“The chain and sprockets on our race sleds take a beating from the harsh landings and constant changes in snow and track conditions. With just routine maintenance, AMSOIL Synthetic Chaincase & Gear Oil prevents us from having parts failures in these areas,” said Judnick.

chain case oil

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Another team favorite? Mudslinger. It provides a protective, non-stick layer of armor against the accumulation of snow.

Mudslinger®

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Will Judnick go big this weekend? Be sure to watch his team live in action this weekend at the Seneca Allegany Snocross National in Salamanca, N.Y.