Maintaining the gear oil in your vehicle is crucial to the life of your manual transmission and differential. If your car is subjected to extreme environments or driving conditions, it may be necessary to change your gear oil more often than your owner's manual recommends. Clunking and grinding noises, difficulty going into gear, or a howling noise from the rear of the vehicle are all signs your gear oil should be changed, or that the level of gear oil is too low. O'Reilly Auto Parts offers a wide variety of gear oil, making it easy to choose the right lubricant for your car's manual transmission or differential.