yep.....dont forget that petroleum companies (certainly in Europe) change their fuels according to season. Bear this in mind when you are tuning an engine.

e.g. in a cold winter the 98 ron petrol is "richer", or of a higher octane, in the winter than in the Summer, to aid starting.
I used to do proddy racing with "pump" fuel in the regs. so i know......
basically, as we are in a heatwave in the UK at the moment (long may it last!) if you tune your engine now, it will certainly be running alot "richer" in the winter, when all the petrol suppliers change their mixture, to aid a cold start.
Also the Esso/shell/Elf in particular, petrol normally is of a better quality than the supermarket, who use a number of petrol suppliers (the cheapest at the time of purchase).

incidentally, I also know that the French Winter mix is "richer" than here in the uk at the same time of year. (the Alps???).....hope this helps.....