interesting thing i have found with older vehicles, some run better on high octane gas, i had to put octane booster in my f-250, it had a stock 460 with 360 000km on the clock, would run like CRAP on regular with no booster... (booster was cheaper than premium)
conversely, my dad used to have a 68 mustang that would purr on 87 octane, but would only run in 2 throttle positions with anything higher, idle would be fine, and WOT would be fine, anything in between it would sputter and give problems... this, of course, is not normal operation, but it does happen to some vehicles, i have no attempt for an explanation for this phenomenon though...