It is safe to use regular IMO, that is what the knock sensors are for. I have used 87 regular in my 3.0 on occasion, but I do notice a performance decrease. I use 89 (plus) 95% of the time, with absolutely no performance decrease or fuel consumption increase over 91 premium.
If you think back to (non-modified) vehicles built prior to the onset of engine management computers, they didn't ping all the time. They could ping on a hill, for instance, at a certain throttle setting, if the AKI was too low for the conditions. Usually it indicated deposit buildup which caused hot spots. If the X5 computer is retarding the timing for the occasional onset of pre-detonation, it isn't doing it all the time but rather under certain conditions of engine load.
Remember that BMW specifies 91 to cover themselves, given the very wide range of fuel qualities in different regions.
The 89 in my area may be of a higher quality than in other areas. I know that when I travel and don't know the local suppliers, I am safer using 91. In those instances, it isn't worth the hassle.
I also agree with the other posters that there is little cash saving. In my area, 89 is $0.06/litre cheaper, and with a tank of 80 litres that is under $5.
Now for a question: How many here believe BMW on the 91 octane issue, but don't believe BMW on the 15,000 mile oil change interval?
