Originally Posted by Grovsnus
Again, what gasoline are you guys using? The newer more efficient engines are more finicky than the older ones, and when BMW says 91 (US/Canada) or 95-96 (Europe), they really mean it.
A problem is that 91 is unavailable in large parts of the US, and mixing 89 and 93 is a hassle.
While you can certainly drive it on 89, the engine will adjust by delaying the spark, which gives worse fuel economy and somewhat reduced power output. So if you have been filling with 89, worse fuel economy is expected. If you've used 91 as recommended, the reason is elsewhere.
The quote in the manual is actually:
Recommended fuel quality
BMW recommends AKI 91.
Minimum fuel grade
BMW recommends AKI 89.
I always understood that to mean you should run AT LEAST 89, but the higher the better. You mention the unavailability of 91 and mixing 89 and 93 together. For me, if 93 is available I am going with that. And on the rare occasion that something like 95 is available, that is what I would choose.
I can't think of any reason the car would rather 91 over 93.