wouldn' use the supermarket brands they come in from overseas and use all sorts of additional chemicals, hence the big
problem earlier this year with cars that used supermarket.
BP / shell etc use home grown petrol as for super unleaded, very doubtful it makes any difference i have used both normal and super on my LC and apart from the extra cost no differece in performance or how much you can get out of the tank, think it maybe a bit more relevant for the more powerful vehicles, M3 etc.
Also i saw on the news this week some big motoring bod did a study on the difference runnin on super and non super over a period of time and it concluded that apart from spending a whole lot of dosh more, there wasnt much benefit, gains in tank was only around 6 Miles!!!! but like people say some appear to notice the difference and some dont, down to your own choice i reckon, i would rather keep me money in me pocket if car running fine.