Tell me you're kidding. In Cali we get the crap 91RON stuff as our "super." It barely does the job and I'd happily pay another 30cents to get 93 over the $4.19 I pay for my this trash. Look, in Europe they pay $8 gallon to fill an M5 which costs 40% more to begin with so we're clearly not in the worst of situations.Also, will it really hurt the car to put in lower octane, with the gas prices skyrocketing???
Anyhow, it's your car so do what you wish but the fact remains that if your the owner then putting less than 91 in an E60 M5 is the epitome of bad decision making.