Gear Oil
A high-viscosity lubricant formulated specifically for manual transmissions, transfer cases, and differentials.
Gear oil is a highly specialized lubricant designed for use in manual gearboxes, transfer cases, and differentials. It has a much higher viscosity than standard engine oil to protect heavy, high-load gears and ensure smooth power transfer. Gear oil typically has a strong, distinctive sulfur-like odor due to extreme pressure (EP) additives.
The EP additives in gear oil, such as sulfur and phosphorus, form a protective chemical layer on gear teeth to prevent metal-on-metal contact under heavy loads. Gear oil is classified by the API (American Petroleum Institute) using GL ratings, with GL-4 and GL-5 being the most common. Using the wrong type of gear oil can cause poor shifting or even chemically corrode brass synchronizer rings in manual transmissions.
Differential and transmission gear oil degrades over time due to heat, friction, and moisture contamination. Regular fluid changes, usually recommended every 30,000 to 60,000 miles, remove metal wear particles and restore protection, preventing expensive gear wear and bearing failures.