My 2006 A4 2.0T is proof that low octane fuel is bad for your Audi. Until recently I would typically mix it up with mostly 89 and a bit of 87 here and there depending on gas prices. Oh was that a bad idea! Trust the 91!!! I bought the car in April 08 after owning a series of American cars. I didn't really think lower octane fuel would damage the motor, other than your typical lowered gas milage and slightly timid performance but boy was I wrong. When I first bought it the first owner must have been doing the same thing because the car was really hesitating upon initial acceleration, especially with the a/c on and when it was hot out. It was almost like I was trying to take off in 4th gear although it's an auto. Once the mph met the appropriate rpm it would take off but the lug was unbelieveable and embarrassing! One radiator, lines, TCM, and tranny later the problem was still not fixed. Thank god it was still under warranty because other issues were addressed but they could not figure it out. Until just a few days ago when my CEL came on after I punched it getting on the freeway. The car was really lacking power and still lugging at initial acceleration. Finally it had thrown some codes! And guess what? Both of my knock sensors were taken out, plus a fuel pressure sensor. For how long it had taken to get to that point, I don't know. I know this is a novel but let this be the final word in the big octane question. 91 PREMIUM FUEL ONLY!!!!!!!!!!!!!!!!!!! IT IS NOT AN OPTION!!!! Although you would think the ECU would account for the lower octane and retard the timing....it didn't! The knock sensors were taken out because of it and the motor went into safe mode accounting for the major poor preformance. Now that they have been replaced, including the fuel it runs great!! Again, 91 only!

 

Well my wife was really weird about only putting premium in it and I said that it was pretty much required.. Well I did the math just to prove its not that much more and if you fill up your whole tank (bout 14 gallons) it comes out to like $2 more... So it isn't really a big deal...

 

ever since i started this post i have put nothing but premium in my a4. thanks to all of you based on your input. and i can tell you that my engine has been nothing but trouble free. i also try to stick with one brand too. shell is my fav so far.