I prefer the premium fuel as well but I'm not sure why, with the Shell V-Power I've noticed that sometimes the car takes a few goes to start in the morning. However with BP ultimate the car start first time every time
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I also have the PBX installed in my 325i and the throttle response is so much better than stock! Not sure if other PBX owners experience this but sometimes it feels like the PBX switches off so I have to reset it then all is good again.