I would be surprised that it would adjust at all. My u understanding is that the engine is tuned to work optimally with a certain fuel grade and performs better or worse with different grades. At some point, a low octane fuel will make the engine underperform and maybe even give problems. But on the other spectrum, there may not be a point in using gas with overly high octane if the engine is not tuned to take advantage of it, however unless you put jet fuel you shouldn't be harming the engine either. I suppose that finding the right octane fuel is best, without any overkill. For a 1.8t, I have been recommended 91 octane (in Canada). Supposed to actually make a little more power than 87, while even reducing the minimal lag at takeoff from stop. Can't wait to burn through this tank and try. However, I have been told that there is a slight adjustment period until you actually notice a difference, need to fully burn out the older fuel and clean up the system with the higher octane fuel before you can see a difference.
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