[Delta0311]

Now what do the egineers at BMW say (you know the people that built the engine and did the research)

Super Premium Gasoline/AKI 91
This gasoline is highly recommended.
However, you may also use gasoline with less
AKI. The minimum AKI Rating is:
> 328i/xDrive: 87
> 335i, 335i/xDrive: 89.
If you use gasoline with this minimum AKI Rating,
the engine may produce knocking sounds
when starting at high outside temperatures.
This has no effect on the engine life.
Do not use any gasoline below the specified
minimum fuel grade. Otherwise, the
engine could be damaged.<
Use high-quality brands
Field experience has indicated significant differences
in fuel quality: volatility, composition,
additives, etc., among gasolines offered for sale
in the United States and Canada. Fuels containing
up to and including 10Ξ ethanol or other
oxygenates with up to 2.8Ξ oxygen by weight,
that is, 15Ξ MTBE or 3Ξ methanol plus an
equivalent amount of cosolvent, will not void the
applicable warranties with respect to defects in
materials or workmanship.
The use of poor-quality fuels may result in
drivability, starting and stalling problems
especially under certain environmental conditions
such as high ambient temperature and
high altitude.
Should you encounter drivability problems
which you suspect could be related to the fuel
you are using, we recommend that you respond
by switching to a recognized high-quality brand
such as gasoline that is advertised as Top Tier
Detergent Gasoline.
Failure to comply with these recommendations
may also result in unscheduled maintenance.


So if you have a turbo charged BMW they HIGHLY suggest you don't use anything under 89 octane.