I've seen this question about gas raised many times in different places in the past. The major point that I've seen is that, if you care about your car, you should ONLY buy Premium grade gas.

The Top Tier Gasoline link is interesting; hadn't seen that before. Again, based on previous info that I've seen, I have always liked Chevron and, if I have a choice, that's always my first choice.

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2008 X5 4.8i | Wheels: Summer> style 336 20" RFT; Winter> style 214 Ferric Grey 20" RFT | Aero kit | ESS E-Flash tune | V1 | LCI Rear Tail Lights | Coding completed | Engine replaced at 90K