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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.

The Difference Between GL-4 and GL-5 Gear Oil

The Difference Between GL-4 and GL-5 Gear Oil

The main difference between GL-4 and GL-5 gear oils is in the level of extreme-pressure (EP) additives.

It’s really very simple. They are not interchangeable and GL5 didn’t replace GL4.

by Eddie Yorkman,|July 5, 2022

If you’ve ever purchased gear oil, or spent any time looking at gear-oil specifications, you have undoubtedly come across the API GL-4 and GL-5 specifications. You may have wondered, what is the difference between GL-4 and GL-5 gear oil? Where can I buy GL-4 and GL-5 gear oil?

Severity – Hypoid gears vs Helical

The American Petroleum Institute (API) classifies gears using GL ratings, ranging from GL-1 to GL-5. Higher GL rating numbers represent more severe applications; GL-1 is the least severe and can usually be satisfied with a motor oil, while GL-5 is the most severe GL rating.

EP additives

The main difference between GL-4 and GL-5 gear oils is in the level of extreme-pressure (EP) additives. EP additives effectively protect gears by providing a sacrificial layer on gear surfaces; these additives are essential for protecting gears operating under heavy loads and extreme temperatures. A typical GL-5 gear oil provides about twice as much EP additive in its formulation compared to a GL-4 gear oil.

Where are GL-4 and GL-5 gear oils used?

GL-4 gear oils are typically recommended for spur and helical gears found in manual transmissions and transaxles operating under moderate speeds and loads.

GL-5 gear oils are typically recommended for hypoid gears in automotive axles operating under high-speed, high-load conditions.

What type/weight gear oil should I use?

For top protection and performance, it is best to consult your owner’s manual/Shop manual for the manufacturer’s recommendation. There, you will find both the specified viscosity (75W-90, 75W-140, etc.) and specification (API GL-4, API GL-5, API MT-1, etc.) recommended for your application. Almost all hypoid gear sets require GL-5 due to the need for significant extreme-pressure protection.

Note the 2nd number is the KEY weight under operating temperatures and conditions. 990, 110, 140, 250, etc..  For example our 75W-80 covers and overlaps the recommendations where 80W-90, 85W-90 are written.

What’s the best gear oil for my vehicle?

AMSOIL SEVERE GEAR® Synthetic Gear Lube excels in protecting gears and bearings from the rigors of severe-service operation. By design, it resists breakdown from high heat, preventing acids and maintaining a barrier to prevent metal-to-metal contact while conditioning seals. Its wax-free construction also improves cold-flow properties, improving fuel economy and cold-weather performance.

• Superior film strength
• Controls thermal runaway
• Protects against rust and corrosion
• Helps reduce operating temperatures
• Maximum efficiency
• Long oil, seal and equipment life
• Flexible easy-pack for clean, fast installation

SEVERE GEAR is recommended in differentials, manual transmissions and other gear applications requiring an API GL-5 gear oil. It can also be used in axles where an API GL-4 lubricant is recommended.


Is High-Mileage Motor Oil a Gimmick?

Is High-Mileage Motor Oil a Gimmick?

A purpose-built high-mileage motor oil can help improve the longevity and performance of high-mileage vehicles.

_by Keith Montgomery|October 25, 2023

In the last decade, the automotive lubricant industry has seen high mileage motor oils emerge and gain popularity for vehicles with more than 75,000 miles (120,000 km) on the odometer. These motor oils are advertised to combat the challenges that high-mileage engines face. But do they really work?

The truth is, high-mileage engines do have unique needs and could benefit from a motor oil designed specifically to address those challenges.

Challenges facing High Mileage Engines

Here are a few of the most common challenges facing engines with an unknown maintenance history or used typical quick lube oil sold from tanks:

  • Increased oil consumption
    Oil consumption is common among non-Amsoil engines. Over time, they experience increased wear, leading to greater oil consumption. Using a suitable high-mileage motor oil that is thermally stable and resists volatility can help prevent oil consumption from occurring. (hint – there is only one)
  • Seal leakage
    Engines with aging seals are more prone to oil leaks. An aging seal loses flexibility, hardens and no longer effectively serves its purpose. A well-designed high-mileage motor oil provides a boosted dose of seal conditioners to restore and recondition aging seals and help stop leaks.
  • Sludge and deposits
    Over time, engines experience increased sludge and deposit buildup. This can be due to oil breakdown from high heat, low-quality oil, excessive idling or insufficient combustion of fuel. A quality high-mileage motor oil can provide enhanced detergency, removing deposit buildup to help restore peak performance.
  • Compression loss
    Engines can lose compression as cylinders and piston rings wear, eventually leading to reduced horsepower and increased blow-by. A proper fluid film helps counteract compression loss by sealing combustion chambers to maintain performance. A purpose-built high-mileage motor oil is formulated to maintain viscosity and provide proper film thickness in extreme operating conditions.

High-mileage vehicles can still provide reliable performance.

The above challenges really do hinder vehicle performance. To protect your engine’s performance and help extend its longevity, it’s important to take steps that help counteract those issues. Our testing suggests that some companies are changing their labels to appeal to customers with high-mileage vehicles but not changing the oil inside the bottle.

Purpose-Built Protection for High Mileage Engines

AMSOIL 100% Synthetic High-Mileage Motor Oil is specifically designed for the unique demands of high-mileage engines, helping extend the life of your vehicle. It delivers purpose-built protection at a time in your vehicle’s life where it could benefit from more-robust maintenance – including boosted detergency and an additive package that fights oil breakdown and leaks.

This may sound too good to be true, so to prove it, we subjected it to independent testing. The result: AMSOIL 100% Synthetic High-Mileage Motor Oil reduced sludge by 67%,* demonstrating its cleaning power that helps engines last longer and perform better.

AMSOIL 100% Synthetic High-Mileage Motor Oil significantly reduces engine sludge.

Trust AMSOIL 100% Synthetic High-Mileage Motor Oil to help breathe new life into your high-mileage engine.

Would I need high mileage motor oil if I started with Amsoil in the first place?

The answer is NO.. All of these issues would not be something needed to be considered if you switched to AMSOIL Signature Series after initial engine break in..

This oil was created for customers who bought a vehicle which didn’t use AMSOIL all these years or who used the questionable oil offered in these “Free oil changes from the dealership” which have no quality standard in their oil what so ever.


How Do I Switch from Conventional to Synthetic Oil?

How Do I Switch from Conventional to Synthetic Oil?

You don’t need to do anything special before you switch to synthetic oil after having used conventional oil.

_by John Baker| Aug 11, 2023

To borrow a famous slogan, just do it. You don’t need to take any special steps before you switch to synthetic motor oil from conventional oil.

There is still some confusion about changing to a different type of oil in vehicles, particularly older models that have accumulated many miles. A small group of ill-informed individuals in garages and on blogs still cling to old beliefs that synthetic motor oil causes roller followers to “slip or skid,” or that switching to synthetic motor oil causes issues in older engines, particularly oil leaks.

Facts have since replaced these outdated ideas about synthetics causing engine problems.

AMSOIL Formulations Are The Exception To The Rule

More people than ever are making the switch to synthetic oil because properly formulated synthetics offer benefits well beyond what conventional and synthetic-blend products provide. Many new passenger car and light truck vehicles come factory filled with synthetic lubricants – not just high-performance sports cars.

AMSOIL for decades have addresses those old rumors regarding the negative effects of switching to synthetics, most of which were never true.

There are a Couple Ways to Switch to Synthetic Motor Oil for the First Time.

  • Jump right in and simply change the oil. Take your vehicle to your mechanic or the nearest quick lube and ask for synthetic motor oil. Or, dust off your drain pan and do it yourself. You don’t have to do anything special to your engine first, and the synthetic oil isn’t going to cause problems in your vehicle that weren’t already there, such as leaking seals.
  • Flush the engine first. Though not required, an engine flush more immediately removes sludge and debris accumulated over time. It will also stop or significantly reduce oil consumption. Many say it improved mileage due to ring sludge.

The main difference between flushing your engine and a normal oil change to synthetic oil is the rate at which the built-up debris is removed from older engines.

If you change to AMSOIL synthetic motor oil without flushing, for example, the detergents in the motor oil will clean accumulated deposits from the engine over subsequent oil changes.

When you use AMSOIL Engine and Transmission Flush, the cleanup is more immediate and helps ensure the engine is clean and free of any accumulated contaminants.

If the engine has a sketchy maintenance history and you want to give it a fresh start, we recommend using AMSOIL Engine and Transmission Flush before you switch to synthetic motor oil.

Ready to make the switch to synthetic motor oil? Check out our Product Guide for the right oil for your vehicle.

Vehicle Look-up: Match AMSOIL Products to your model


Your Manufacturer’s Warranty is Secure

Your Manufacturer’s Warranty is Secure and better protected with AMSOIL

AMSOIL lubricants maintain your factory vehicle and equipment warranties.

_by David Hilgendorf October 24, 2022

According to the Federal Trade Commission, manufacturers or sellers may make promises to stand behind the products you purchase. Any warranty offered must be available to read before purchase, so be sure to understand the details of what is covered before you buy.

Vehicle and equipment warranties often cover the expensive costs of labor and parts necessary to fix certain product defects or malfunctions within a predetermined amount of time or usage. It is important to know that your manufacturer’s warranty is secure for peace of mind.

Whether investing a few thousand dollars in lawn equipment, tens of thousands on powersports vehicles that may only be used a few months out of the year, or buying a new car or truck for daily use, manufacturers know these are big investments. They put a lot of focus on warranties to build consumer confidence and want you to buy their brand of parts and lubricants instead of aftermarket products.

Feeling pressured to use their products?

Some original equipment manufacturers (OEMs) falsely claim that you must use their brand of parts and lubricants or risk voiding your factory warranty. OEMs may even attempt to make you feel like they control what you can do with your vehicles. This is one of the biggest hurdles faced when recommending AMSOIL.

Thankfully, in America, the Magnuson Moss Warranty Act [15 USC Ch. 50 §2302(c)] protects your freedom to choose any brand of aftermarket parts and lubricants and your right to extend oil-change intervals beyond what is recommended in your owner’s manual, without voiding your warranty. Similar protections are available in Canada.

U.S. Federal law Protects you

Here are key points to remember for maintaining your factory warranty:

  • It is illegal to tie warranty coverage to the brand of parts or lubricants you use unless the manufacturer provides the parts or lubricants free of charge.
  • You have freedom to choose how to protect your vehicles and equipment, including extending recommended drain intervals.
  • Manufacturers can’t deny warranty coverage without proving an aftermarket part or lubricant caused a failure.

If anyone tries to tell you differently, contact the FTC at

Tips to avoid warranty issues

No one wants to file a warranty claim. It’s a hassle and can take months to resolve. Here are a few tips to help smooth the process should you ever need to use your factory warranty.

  • Read your warranty. Familiarizing yourself with what is and is not covered is worth your time.
  • Period of coverage. If your vehicle or equipment acts up, have it looked at before the end of the warranty term.
  • Service records and receipts. No matter who performs the service, providing proof that you maintained your vehicle removes the easiest way for manufacturers to shirk responsibility.
  • Speak up. If you think a service adviser denied your warranty claim unfairly, ask to speak with a supervisor. If the dealer does not resolve the issue, try another dealer, or contact the manufacturer. If you still aren’t getting results, contact your attorney general, local consumer protection office or the FTC.
  • Avoid aftermarket warranties. Many are scams. If a dealership is selling one please check the BBB looking up the company first! I can bet there are a lot of angry customers! If you think it’s legit remember it’s a contract and NOT a warranty so be sure to have them add in that you are going to use AMSOIL on your terms.

Strong products, strong warranty

AMSOIL designs high-performance replacements for manufacturer-branded lubricants that keep your factory warranty intact.

AMSOIL developed Warranty Secure branding to simply and effectively reinforce your right to use AMSOIL synthetic lubricants. We use clear, direct language to remind consumers that the law is on their side and to dispel fears generated by misleading or false warranty claims from any manufacturer, dealer or mechanic.

Because this issue is more prevalent in the powersports market, the Warranty Secure icon appears on all our powersports products, including AMSOIL Synthetic V-Twin Motorcycle Oil and ATV/UTV oils.

AMSOIL also offers its own Limited Warranty, offering an extra measure of protection. While some oil companies guarantee their products only for a specified mileage, we stand behind our products for as long as you use them.

Runs on Freedom™ Snowmobile Warranty

Additionally, AMSOIL offers the revolutionary Runs on Freedom Snowmobile Warranty offering complete peace of mind. Simply use AMSOIL INTERCEPTOR Synthetic 2-Stroke Oil exclusively in your new sled and we’ll cover any engine repairs for up to two years or 5,000 miles (8,000 km). Read the terms and conditions and register your sled today!

No other aftermarket snowmobile lubricant manufacturer offers this robust of a warranty. We’re so confident in our products, we’ll cover any repair to your snowmobile engine in the unlikely event of an engine failure. This means AMSOIL may end up fixing defective OEM parts that fail for reasons unrelated to our oil, but we’re more concerned about protecting our customers and their machines.

As millions of satisfied AMSOIL customers know, using AMSOIL synthetic lubricants or practicing extended drain intervals does not void vehicle or equipment warranties.

AMSOIL stands behind its products and the people who use them. If the OEM claims use of AMSOIL products violates the warranty, contact AMSOIL Technical Services (715-399-8324) or visit