I used to think the same thing, my Altima is "sensitive" to different oil brands/types (or I just feel a difference). The worst it ever felt I was using Amsoil 5w30 ASL. The second worst is Mobil 1 EP, so I started thinking it was something to do with the true synthetics. It has to be something to do with viscosity though because if I use M1 0w40 I can't tell the car is running sometimes. Ultra is also pretty good too, so I have no idea.