I had a 530 and installed a Dinan chip which made a check engine light come on when coasting down a long grade . Changed to Conforti which was better. Mark D'Silva has a chip for these now at a much better price and I hear his chip is as good or better than any of the others.
For what its worth, I do not know of any BMW's that do not require premium fuel. Thus, I cannot understand why any enthusiast would run anything but premium in a high performance machine. In the US, generally 91 to 93 is available and 89 is usually mid-grade. Many of these cars do not have a knock sensor to retard the timing. Just a few thoughts.
Mike Holbrook
Meridian, Idaho
1992 535im, 17", Euro M5 Throwing Stars 8's & 9's, FK-451 235/45s & 255/40s, M5 Sway Bars 25/20, Conforti chip, Lowes Ram Air, glass sunroof