More Performance, Less Wear: Why Gear-Oil Compatibility Matters

Many people overstress their vehicles pulling trailers, campers or boats, sometimes exceeding their rated towing limits in sweltering heat. Commercial vehicles often spend their lives in severe service. Severe-service conditions place extreme pressure on transmissions and differentials, making the proper selection of gear oil essential for maintaining the performance, efficiency and longevity of vehicles.

GL-4 vs. GL-5

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 robust for managing severe applications.

The main difference between GL-4 and GL-5 gear oils is the level of extreme-pressure (EP) additives included in the formulation. EP additives protect gears by providing a sacrificial layer on gear surfaces. They are essential for protecting gears operating under heavy loads and extreme temperatures.

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.

GL-4 Gear Oil

GL-4 oils are typically recommended for spur and helical gears found in manual transmissions and transaxles where moderate speeds and loads are expected. They contain a balanced amount of extreme-pressure (EP) additives for smooth gear operation without causing undue wear to softer, yellow-metal components like brass or bronze synchros. This makes GL-4 the go-to choice for specific gearboxes requiring compatibility with sensitive components.

GL-5 Gear Oil

GL-5 gear oils are typically recommended for hypoid gears in automotive axles and differentials where high-speed and high-load conditions are common. GL-5 oils have nearly twice the EP additives of GL-4 oils. These advanced additives form a sacrificial barrier, protecting against wear in high-load regions. This formulation makes GL-5 oils indispensable for vehicles subjected to towing, off-road use or other extreme conditions.

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.

Advanced Technology

Modern vehicles boast more horsepower and torque than ever before, subjecting their differentials to increased levels of stress and heat. Thermal runaway has become a true threat. Gear oils must provide adequate wear protection in extreme conditions, while also providing maximum fuel efficiency. To protect these vital components, it’s essential to install the recommended gear lube for your vehicle. To maximize performance and protection, AMSOIL SEVERE GEAR provides technology that has been tested in the most extreme applications on the planet. For those who demand the best, AMSOIL SEVERE GEAR offers peace of mind backed by cutting-edge science and proven durability.

This article is part of our Transmission and Gear Oil guide. See every article in the guide.

Related Reading

• GL-4 vs. GL-5 Gear Oil: What’s the Difference?
• What Is Hypoid Gear Oil? How It Compares to Regular Gear Oil

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• GL-4 vs. GL-5 Gear Oil: What’s the Difference?
• What Is Hypoid Gear Oil? How It Compares to Regular Gear Oil
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