They used to do winter and summer blends in California. Not that one had any choice in the matter but the "oxygenated" fuels got you poorer mileage with the benefit of cleaner emissions. They have finally phased out the MTBE additive that made such a mess of lakes and such when used in marine applications.
As far as your mileage issue goes, have you actually checked it in the standard way? Sometimes the sensors in the tank will degrade somewhat and the readings can change slightly. Going from "Full" to "3/4" tank in a certain amount of miles is probably not the most scientific way to judge fuel economy. Top off the tank, zero out the trip odometer, and drive the car. Next time you fill up, check the trip odometer and do a little math. I don't know if you ever did that back when the X5 seemed to get better mileage, but it would give you a baseline one way or the other.



Gar