I recently moved back to the States from Germany with my U.S. spec. '99 M3. In Germany, I used Esso premium gasoline - their octane rating translates to about a 95 here. Since I've been back, I've had rough idling after cold starts, and hesitation until the car warms up. The dealer recommends trying different premium gasolines. I have used Texaco, Mobil and Exxon premiums (92-93 octane) with about the same results. I also have Chevron and Union 76 in my area. Any suggestions on the best, and why?