Well, I certainly feel sorry for you Californians and other Westerners that are limited to 91 octane "oxygenated" unleaded fuel, during certain times of the year I believe.
On a driving trip over there recently, I clearly noticed a performance difference with 91 oxygenated in my 300E, versus the 93 octane you can get here in Texas ... not to mention the price difference. While I was there, super unleaded was about $1.88 a gallon ... here it averages about $1.48 or so. I dare to guess what performance the regular unleaded would've given me.
Now, I have no clue if the difference was due to the octane, or in the oxygenation, but the performance difference was definitely there. Climbing mountains, I also heard an occasional ping or two.
I'll have to side with BlackMercedes. You get what you pay for. I only use premium unleaded as stated just below my fuel gauge needle.
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi