The main difference between GL-4 and GL-5 gear oils is how much extreme-pressure additive has been included in the mix. GL-5 oil has roughly twice as much of the EP additive as GL-4.
Gear oils all include some level of sulfur or phosphorous additive. This additive coats the gears and wears off as they rub against each other. This process protects the gears from wear. GL-4 is typically used for vehicles with a high level of yellow metal, because the additive corrodes such metals.
The American Petroleum Institute sets the guidelines for automotive gear lubricants according to their intended use. The API category GL-4 for gear oils designates the lubricant as appropriate for spiral-bevel and hypoid gears in automotive axles. The car should be operated under moderate speeds and loads. GL-4 oils are appropriate for selected manual transmission and trans-axle applications.
The API category GL-5 for gear oils designates the lubricant as appropriate for hypoid gears in automotive axles. The lubricant is sufficient for high speed, low speed and high-torque conditions. The U.S. Military qualifies some oils as MIL-L-2105D, MIL-PRF-2105E and SAE J2360, all of which meet the requirements of GL-5 gear oils. However, SAE J2360 gear oil actually out-performs GL-5 brands.