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

Written by Charleen

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.

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Signature Series Beyond Their So Called Clean

Written by Charleen

Signature Series cleans up the competition

In general, the higher an oil’s TBN, the better its ability to neutralize contaminants such as combustion by-products and acidic materials. Higher TBN oils neutralize a greater amount of acidic material, which results in longer oil life and cleaner engines.

TBN levels decrease as the oil remains in service. When the level reaches a point where it can no longer protect against corrosion, the oil must be changed.

The TBN Test

The Total Base Number Test (ASTM D2896) uses a series of chemical reactions to measure the alkaline additives in a motor oil. We recognize the value of formulating motor oils with high TBN, and AMSOIL Signature Series Synthetic Motor Oil features the highest TBN of the oils tested.

AMSOIL Signature Series is fortified with a heavy treatment of detergent additive and it delivers 30% more acid neutralizing power1 than Mobil 1, and 36% more than Royal Purple, helping engines to stay cleaner, longer.

The AMSOIL Advantage

AMSOIL synthetic lubricants feature a large dose of quality additives that consistently deliver high TBN for the life of the oil. They neutralize acidic contaminants and keep them in suspension to maximize engine protection. AMSOIL lubricants use detergent and dispersant additives to significantly reduce sludge and carbon deposit formation. In the Total Base Number Test (ASTM D2896), AMSOIL Signature Series 5W-30 Synthetic Motor Oil demonstrated the highest TBN of the oils featured in the test, helping it deliver reliable protection for extended drain intervals. One thing the test doesn’t show is our detergent lasts longer. You can see that for yourself when you pull your own oil analysis.

Aftermarket Additives – Did you know?

Just by adding your aftermarket oil additive meant to help with “something”, these additives are used because the user believes it offers an additional attribute which the base motor oil does not offer. With AMSOIL you never need to do this. Anything offered on the market was reviewed often decades ago and if it offers some advantage it will be in the AMSOIL product. If not you risk neutralizing additives such as the detergents which make up the largest part of the price of AMSOIL other than the base oil.

There are several additives which took decades of knowledge to get them to perform together without nullifying the benefits. That’s what aftermarket additives do, they often make a good oil worse than bad. After the additive is added you may have just been better off getting your oil at the dollar store or just using the used oil out of your sisters old Saturn.  (Assuming it still runs because she didn’t use any of these caustic additives)..

Research the brand and even their address

Note – research the company you buy the additive from. One very big one which in the 60’s had their name on Indy cars, etc has been on the FTC’s warning site more than once and has actually been bought and sold no less than 6 times. Each time it becomes a totally different product. Others actually do some good  but at the same time are causing a greater negative in the oil somewhere else.

1Based upon independent testing of Mobil 1 Annual Protection Full Synthetic 5W-30, Royal Purple High Performance 5W-30 and AMSOIL Signature Series 5W-30 in ASTM D2896. Oils purchased 05/03/18.

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