I document every fill up, gas type, octane, amount, cost, mileage. Over the last 12 months I can say using 87, 91 or 93 octane makes no difference in miles per gallon. I feel (IMHO) that I get better performance with the highest octane to a point, 93, I've tried 93.5 and I couldn't tell any diff. I do notice trends in mileage from different gas stations. Exon and BP have consistantly given better MPG over Shell, Chevron, Phillips 66 and Entec. I couldn't say if it's the car liking those blends or those blends are consistantly better but I keep records and charts to show there is a difference. Since I started using FP and LC I'm getting almost a 1% increase in MPG regardless of the gas type. My car is only 1 year old and if it makes a difference in my new bimmer, think of what it could do for a 5 or 10 years older vehicle. I know it sounds like a sale pitch but I'm just expressing my experience using this stuff and want to pass the knowledge to other BMW owners.
Average Jae, not everyone has a modern car! The knock sensors are not a cure all for knock they can only do so much, long term use retarding detonation is being noted as a bad thing for the owners down the road.