You can feed it 91 and above and be perfectly fine.
There are knock sensors that will retard your timing
and do some other things when they sense pre-ignition ( knock)
when you use lower octane gas. But don't use 87 !
In a naturally aspirated motor you can feed it almost anything and
with knock sensors it will be fine .
However when you do this you lose power and have to push the
motor harder negating much of your savings by using the lower grade
87-89 octane gas.
HOWEVER in a TURBO - I would not use anything below 91.
Your power production will suffer dramatically and you just don't want
today's high compression turbo motors struggling with less octane than
they need. Back in the day before computers et al a turbo motor would have
a low compression ratio 7 or 8 to 1 on average. Today
the compression ration is pretty high- which is why we are seeing
such high power being wrung from these relatively small turbo motors.
Bottom line -no less than 91 for a turbo motor. If you chose to drive
a turbo BMW- feed it what it needs to get what you paid for from
the car. Cheers
2013 Porsche Boxster S
2011 Lexus GX460 Premium