I want a 98RON Map and there are only 97RON (BP Ultimate and Esso Super Unleaded) near me. There is a Tesco close to me with 99RON Momentum. I'm always a bit wary of Supermarket fuels but since the Octane rating of a fuel is its anti knock properties should the Tesco stuff be better than the BP Ultimate? It's to go in my 335i so I'm a bit wary of det'ing the engine ('power' and fuel consumption don't bother me, just stability). I know there's a lot of smart guys on here who can put my mind at ease. Cheers