Just to add a bit more to this, I always run my car on Tesco 99 and occasionally V-Power, but when in Wales last week all I could get in was BP Ultimate. After 1/2 tank, under full acceleration, I noticed it surging a bit, still quick, but not as smooth. Once the tank was empty, I refilled with Tesco 99, and 50 miles or so later, good as gold again