I maintain 4 Volvo's that spec the JWS 3309 AT fluid and perform a cooler line exchange every 20k with Mobil 3309. After each exchange there is noticeable, actually surprising improvement in shift smoothness. These are gently driven cars. I have 2 questions: 1) Why is there such and improvement in shift performance using Mobil 3309? Is this by design? 2) Will a full synthetic fluid that meets the JWS 3309 spec work just as well or better? Thanks in advance for your comments.