Confused about the difference between GL-4 and GL-5 gear oils? Using the wrong oil can lead to costly equipment damage.
Not knowing which oil to use in your gears can cause premature wear, reduced efficiency, and even catastrophic failures. Don’t risk your machinery’s performance.
In this post, we’ll clarify the key differences between GL-4 and GL-5 oils and when to use each, so you can keep your gears running smoothly.
GL-4 Gear Oil
GL-4 gear oils are designed for manual transmissions and other gears where extreme pressure and shock loading are not major factors. They contain additives that provide good oxidation stability, rust protection, and anti-wear properties suited for gears operating under moderate loads and speeds.
GL-4 gear oils have a lower concentration of extreme pressure (EP) additives compared to GL-5 oils. The primary anti-wear additive in GL-4 oils is typically zinc dialkyldithiophosphate (ZDDP). The lower EP additive content makes GL-4 oils more compatible with yellow metals like brass and bronze used in synchronizers and bushings in some manual transmissions.
GL-5 Gear Oil
GL-5 gear oils are formulated for gears operating under high loads, high speeds, and shock loading conditions, especially hypoid gears in automotive axles and differentials. They contain a higher concentration of extreme pressure additives compared to GL-4 oils to prevent scoring and seizure of gears under severe conditions.
The primary EP additives in GL-5 oils are sulfur-phosphorus compounds and organic sulfides. At high pressures and temperatures, these additives react with metal surfaces to form a sacrificial coating that prevents metal-to-metal contact and scuffing. However, the higher sulfur content can corrode yellow metals in some manual transmissions.
Extreme Pressure (EP) Additives
Extreme pressure additives are crucial components in gear oils that prevent metal-to-metal contact, scuffing, scoring, and seizure of gears under high loads, especially shock loads. They form protective films on gear surfaces through chemical reactions at high pressures and temperatures.
Common EP additives include sulfur-phosphorus compounds, organic sulfides, and chlorinated paraffins. The concentration and type of EP additives differ between GL-4 and GL-5 gear oils based on the intended application and severity of operating conditions. GL-5 oils have higher EP additive content to withstand more extreme conditions than GL-4 oils.
When to Use GL-4
- Spur and helical gears: These gears have teeth parallel to the shaft axis (spur) or at an angle to the axis (helical). They operate at lower sliding velocities and generate less friction heat compared to hypoid gears. GL-4 oils provide adequate protection for spur and helical gears in most automotive transmissions.
- Hypoid gears under severe service without shock loading: In some cases, GL-4 oils may be used in hypoid gears operating under severe service but without extreme shock loads. However, GL-5 oils are generally preferred for hypoid gears, especially in automotive axles.
When to Use GL-5
- Hypoid gears in automotive axles operating under high-speed, high-load conditions, including shock loads: Hypoid gears have teeth that are curved and not parallel to the shaft axis, allowing the pinion to be off-axis relative to the ring gear. This design accommodates larger pinions for more torque capacity but generates significant sliding action and friction heat.
- Differentials, especially those with hypoid gears: Automotive differentials, particularly rear axles, often use hypoid gears to transmit power at right angles while allowing speed differences between the left and right wheels. GL-5 oils are designed to protect these gears from the extreme pressures and potential shock loads encountered in differential applications.
- Manual transmissions with synchromesh friction modifiers: Some modern manual transmissions require gear oils with specific friction modifiers to ensure proper synchronizer operation and shift quality. Certain GL-5 gear oils are formulated with these synchromesh friction modifiers and are recommended by the vehicle manufacturer for such applications.
Can GL-4 and GL-5 Gear Oils Be Mixed
Mixing GL-4 and GL-5 gear oils is generally not recommended due to differences in additive chemistry and potential compatibility issues. GL-5 oils have higher sulfur content that can be corrosive to yellow metals like brass and bronze used in some manual transmission synchronizers and bushings. Using GL-5 oil in a transmission designed for GL-4 can lead to premature wear and failure of these components.
Conversely, using GL-4 oil in a differential or axle designed for GL-5 may not provide adequate extreme pressure protection, resulting in gear scuffing, scoring, or seizure under high loads and shock loads.
