All premium Chevron, Shell, or 76, all the time (never Arco or grocery store gas garbage). And I'll sometimes fill with the occasional tank of ethanol-free premium when I happen to be near the grange.

Also, I throw in a bottle of Techron every few thousand miles. And I've been using Lucas fuel treatment pretty regularly too.

ETA: I don't understand why people balk at the price of premium gas. At $.30 more per gallon, a whole 25 gallon fill up will cost you $7.50 more. Seems worth $7 to get the proper performance out of your car.

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2006 X5 4.4i
Sport package, summer package, cold weather package, DSP + Nav
Bought 2/14 with 73k miles