ATF is automatic fluid, a transmission oil used in vehicles with automatic shifting. Unlike transmission oil for ordinary manual vehicles, it also features a friction modifier, making it possible to shift reliably even on automatic vehicles. The performance will deteriorate with use, and you will feel a shock when shifting, and the acceleration will worsen. Therefore, it is desirable to replace it at about 20,000 km. Some products contain almost no additives, while others contain antifoaming agents and antioxidants.