I've been suffering a bit from pre ignition when pulling away to such an extent that it is going into the dealer for a check. Most noticeable at cold and as I drive 90% of the time with the top down I can always hear it. Most of my driving is relatively local so I use the nearby Shell garage to fill up but recently had to use Esso and suddenly it was much improved, not totally cured but much better. Am I imaging things or is there some possible logic in this?