The 556 D5B's are the ones I have also. I swapped out my 3.5 bar (50 psi) fuel pressure regulator a couple weeks ago and went back to stock. I notice my mileage went down with the 3.0 bar and also that I felt I did not have as bright of a response to the gas pedal. Yesterday I changed the 3.5 bar back in and although I have not yet checked the mileage, the response to throttle is back.
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