Hi,
My '01 X5 3.0i 5Spd, ~85,000 miles, mileage began to drop noticeably (from 19.6 to 16) last year. About 6 months later it started throwing fault codes for the camshaft position sensors- first one and then a few weeks later the other one. After replacing them, my car runs MUCH! better and the mileage is back up to normal.
My understanding is that as these fail they cause the car to run rich.

Replacing both sensors is easy- one is easier than changing an air filter as you literally disconnect a harness, unscrew one screw, pull out the sensor and reverse the process. No need to remove anything to get at it. The other involves a bit more work but is easily accomplished in an hour or less I certainly would not pay the dealer $200 in labor for this job, even if you have no mechanical skills. There are instructions somewhere on one of these forums...

Best of Luck,
Randy Hermann