Generally, your user’s manual will provide specific instructions on how and when to change your transmission fluid and filter. A solid recommendation is to change your transmission fluid after the first 5,000 miles, and every 25,000 miles after the initial change. Manual transmissions usually only need to be changed every 50,000 miles, however. If the vehicle in question is used for towing, it is safest to go ahead and change both the fluid and filter every year.