Supermarkets purchase their fuel from the cheaper Eastern block. They have higher levels of conatamenents than other fuels. I will only use Shell Optimax in the RS and if i have to stick anything else in it's Esso Super Plus or as a final resort BP Ultimate. Do use supermarket fuel in the Audi though as I don't really care about it. Anyway sooner it dies the sooner I can transplant the 1.8 turbo lump into it