Yep, I've noticed the cheaper brands of fuel in my old Saab do (or don't do) the same job as BP or Shell too. Some friends think I'm nuts, but I reckon their 98 branded fuel is closer to a 95. There's a noticeable difference in performance overall. So much for 'premium'.I got Tesco fuel (petrol) before - never again.
It may have been a "bad batch" but enough for me to swear off it.