My Seven is a 2.0L 16v Vauxhall giving out about 210 bhp, rather than a K series VVC, but having used unleaded fuel of 95, 97 and 98 have to say that personally little difference has been noticed. I tend to use what I can get hold of but make sure that it is regularly filled with SUL, just in case.I like to hedge my bets! Perhaps you should try the different grades and see what best suits your set up.