The Society of Automotive Engineers (SAE) established a viscosity rating system for gear oil, with higher numbers indicating thicker oils. The “W” that comes right after the SAE number indicates that the oil met an SAE specification for cold-start viscosity. It stands for “winter,” not for “weight.” Oils that have two numbers and include a “W” after the first number, such as 75W-90 or 75W-140 gear oil, are multiviscosity oils. 75W-90 and 75W-140 gear oils perform similarly at colder temperatures, or when the gears first begin turning. When the gears get hot, however, 75W-140 provides better protection against wear.
Consider the operating conditions of your vehicle or machinery. If you operate it in the winter and live in a climate where outdoor temperatures drop below freezing, you may want to use 75W-90 gear oil. If you are an over-the-road trucker, you also may choose 75W-90. This gear oil gives you better mileage because at operating speed it is thinner than 75W-140. If you drive in extreme conditions that cause increased pressure or heat, you will want the extra protection that 75W-140 provides.
Within each of these two categories of gear oil are sub-categories. Some 75W-90 gear oils, as well as some 75W-140 gear oils, are manufactured to handle the high heat and extreme pressure within drive train differentials. These oils usually will be labeled “high performance” or “severe.” Other sub-categories emphasize that they require fewer oil changes. You might chose this type for over-the-road truck differentials, which usually have larger sumps and operate at lower temperatures than other vehicles.
Most 75W-90 and 75W-140 gear oils are synthetic oils that contain additives to keep them from becoming too thick to flow in cold temperatures and too thin to protect in high temperatures. They pass a 20-hour shear test, which requires that the oil does not shear down. This means it does not separate from the parts it is protecting or decrease in viscosity so that it is thinner than the minimum allowed for that grade. Many companies subject their gear oils to other tests to determine their ability to resist oxidation and foaming and prevent corrosion. Compare test results to determine the most suitable oil.