im getting the same on my focus 1.6, i use shell V-Power at 115 a litre and getting about 100miles on 20 quid, however i got this after i fitted my heat wrap on my four branch tubular and didnt put enough exhaust putty on the joint and it very slightly blows (will be sorting it) however, i tightened the joint up slighlty whilst my car was running and my engine idle changed so just check to make sure on that, just an idea.