Late to the party, as usual, so a couple of comments that may not be read or appreciated. No-one has mentioned VALUE, always COST. Do the sums. Super Unleaded cost premium is usually about 10-12p litre more than E5 std UL. So for std UL costing £1.40/litre and your 2.5(?) doing 33mpg, that's costing you 19.29p/mile. Buy Super unleaded at say £1.52/litre, but get 36mpg, each mile is only 19.19p/mile. I reckon I get 4-5mpg more, on average (depending on how much I use the loud pedal).
I have done around 25K a year for the last 5 years in Z3 (2.8), E85 (2.5) and E89 (3.0) and always better the figures above, doing the same trip (500 miles day) on same routes in all weathers. I reckon it costs MORE to use CHEAP stuff - Yes I still groan at the price on the pumps, but when you shop around and find the higher Octane supplies (like Shell at 99 octane), in preference to the 97 grade that some supply, my mpg always better. Only one caveat - if you are only doing little short trips around town then stick with the cheap stuff or there will be no cost benefit. Don't forget the better additives are good for the old ladies that we cherish!