I had one vehicle (the 535i) that made more power on 94 than the specified 91. The fact that our 94 has zero ethanol made it attractive as well. We are now into the realm of forced induction, though, not something that applies to the E53.

Pulling the timing back isn't an on or off thing. And it isn't going to be doing it all the time, unless you are using 85 or something similar. On 91, it may do it hardly every. On 89, it may do it 1% of the time, just as an example. That would mean that for 99% of the time there would be no difference. It may only do it at a certain ambient air temperature and throttle setting. If an owner can't measure a fuel efficiency difference on a lower AKI fuel, then I submit that for all practical purposes any timing retardation is so negligible as to not matter at all. Increased fuel consumption will show up the same time as lower power does.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue