The short answer: The difference is extreme-pressure (EP) additive: a typical GL-5 gear oil carries about twice as much as a GL-4. GL-5 is for hypoid gears in axles and differentials under high speed and load. GL-4 is for the spur and helical gears in many manual transmissions and transaxles under moderate loads. Always use the specification your owner’s manual lists.
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
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?
Where Each One Belongs
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.
While some GL-5 oils claim backward compatibility with GL-4 applications, it’s critical to follow manufacturer specifications to avoid excessive wear on sensitive components. For optimal protection, consult your owner’s manual or use the AMSOIL Product Guides for the manufacturer’s recommendation. These sources will provide the specified viscosity (75W-90, 75W-110, 80W- 90, 75W-140, etc.) and specification (API GL-4, API GL-5, API MT-1, etc.) recommended for your application.
What type/weight gear oil should I use?
For top protection and performance, it is best to consult your owner’s 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.
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.
Thermal Runaway
The extreme pressures and temperatures generated by modern vehicles can lead to a serious condition known as “thermal runaway.” As differential temperatures increase, gear lubricants lose viscosity and load-carrying capacity. When extreme loads break the lubricant film, metal-to-metal contact occurs, increasing friction and heat. This increased friction and heat, in turn, results in further viscosity loss, which further increases friction and heat. As heat spirals upward, viscosity spirals downward. Thermal runaway is a vicious cycle that leads to irreparable equipment damage and ultimately catastrophic gear and bearing failure.
Proven Protection
AMSOIL SEVERE GEAR® Synthetic Gear Lube is engineered to meet the demands of the most punishing applications and extreme environments. SEVERE GEAR is engineered with high film strength for high-load demands. It reduces friction and provides the ultimate protection against wear, excels in extreme temperatures, helps maintain efficiency and far outperforms conventional gear oils. SEVERE GEAR is excellent for all cars and trucks, but it’s especially well-suited for towing, hauling, racing, commercial use and other severe duty.
SEVERE GEAR is formulated with proprietary additives that create an iron-sulfide barrier on gear surfaces, shielding them from wear, pitting and scoring. This unique technology helps ensure that components stay in peak condition, even under the heaviest loads.
How good is it? SEVERE GEAR uses technology that has been tested in one of the most challenging applications on earth – wind turbines. Prior to AMSOIL entering the sector, the wind industry struggled with high maintenance costs and component longevity. AMSOIL helped turn that around, and today we are a significant player in the industry. While our synthetic formulations for wind turbines differ from automotive applications, some of the same pioneering technology that protects turbine gearboxes is used in SEVERE GEAR formulations to protect vehicles.
Video transcript: AMSOIL: Severe Gear Easy Pack Infomercial
Servicing differentials is virtually impossible. The confined space makes fill holes difficult to reach, and industry standard conical bottles are rigid and hard to squeeze. That combination makes for a messy job. Plus, you can’t get all of the gear lube out of the bottle.
So, you have to buy more than you need to complete the job. And engineering your own solution only results in disaster. Not anymore. Introducing the new AMSOIL Easy Pack.
Easily access hard-to-service differentials. Easily squeeze out all of the contents. And easily stay clean while you do it. No more leftover fluid or inconvenient rigid bottles.
No more trouble reaching difficult fill holes. No more mess. AMSOIL Severe Gear in the new Easy Pack. The solution to all your problems.
This article is part of our Transmission and Gear Oil guide. See every article in the guide.
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Start with AMSOIL synthetic motor oil and find the exact AMSOIL products for all your vehicles with the AMSOIL Product Guide.
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Joel Youngman is Lead Technical Writer at AMSOIL and associate editor of AMSOIL Magazine. He develops and edits content covering lubricant technology, product testing, vehicle systems, maintenance practices and equipment applications. Youngman also coordinates magazine articles and Tech Talk columns, working with engineers, product specialists and Communications personnel to maintain technical accuracy, editorial consistency and clear customer communication.
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