75W-90 gear oil is a lubricant designed for manual transmissions, transfer cases, and differentials. This viscosity is ideal for colder climates, while still maintaining adequate high-temperature viscosity. However, if you use your vehicle for hauling, towing, or other stressful driving situations during the summer, 75W-140 gear oil may provide extra high-temperature protection. If you notice clunking or grinding from under your vehicle, check the condition of your gear oil. If it is low, or if the gear oil itself is dark and smells burned, consider changing your gear oil. Always consult your vehicle's owner manual for the manufacturer-recommended gear oil weight. At O’Reilly Auto Parts, we carry gear oil and limited slip additives for most vehicles.