If you own a car with an automatic transmission, then it's very likely that your transfer case contains gear oil. This type of oil is specifically designed to lubricate the gears and bearings inside the transfer case, and helps to keep them running smoothly. While there are many different brands and types of gear oil available on the market, it's important to choose one that's compatible with your particular transfer case. In general, most transfer cases will require a synthetic gear oil with a viscosity of 75W-90 or 80W-90. However, it's always best to check your owner's manual or with a qualified mechanic to be sure. Once you've selected the right gear oil for your car, simply pour it into the fill hole located on the side or rear of the transfer case. Be sure not to overfill it, as this can lead to problems down the road.