If you get better gas mileage at the expense of performance, does that suggest that 87 would be good for a long road trip where power isn't a concern? My fuel door says "minimum 89 octane"... so I can't imagine that I'd be damaging my engine if I used that.
On the other hand, if I'm using the "fuel economy" mode on my JB4 tune, the timing is probably already adjusted without needing to change the fuel grade...
2008 135i Montego Blue sport pkg, premium pkg, Logic 7 sound, iDrive/Nav
Mods completed: JB4+ & FBO || LSD || BMW-P suspension + F/R sway bars || 18" Breyton GTS-R || M3 front bushings || Vogtland camber plates || BMW-P rotors + Stoptech pads |||| Acquired, not installed: BMS WW Meth kit || Logic HVI intake || Mishimoto OCC |||| Shopping for: Summer tires