this is my personal experience i'm basing it off of. When i first started breaking in my car i ran a mix of 110 octane and 93 octane giving me a calcuated value of 103ish octane. I was trying to tune to car to eventually run 93 octane normally. I also wanted to run a higher octane first to be safer. With my wideband, i tuned the motor to run the 103 octane mix. I raised the 93 octane amount and lessened the 110 octane amount on my next fill up giving me a octane rating of about 97 octane. noticed that the a/f ratio didn't really change. It may have changed but i was looking for any changes over 1/2 pt. I then decided to put no 110 octane in and fill up w/ 93 octane. The mixture was calculated to be about 94 octane w/ the remaining amount left in the tank. my a/f didn't change but i was only looking for changes greater than 1/2 point again. I was tuning full throttle only to 13:1 a/f and was only looking if any point was greater than 13.5 and less than 12.5 and didn't get any dramatic changes. The only difference i got was the my motor would knock above 6000 rpm w/ 94 octane and less. 97octane and greater didn't knock above 6000 rpm w/ the same timing. I tried to retard the timing w/ the 94 octane but just couldn't get the knocking out. I retarded the timing so much that it was pointless to rev over 6000 w/ 94 octane. Personally, i didn't have any changes w/ going from higher octane to lower octane as for a/f ratio. Nothing noticable at least. The only difference i had was timing. The timing had to be changed. Now I also didn't go to a dyno to see which made power. I wasn't worried about power at the time, i was only worried about the motor running safely.