from what i've noticed, when time comes close to an oil change, the gti
gets better slightly gas mileage (from 300 miles on 13 galons to 320 on
the same amount). maybe something to do with less oil to slush around.
right after the oil change, it goes back to normal.
if you're really worried, go get your codes scanned at the dealer. the
ecu will say if there's something wrong...
did the dealer fill your gas tank? maybe they put in 91 instead of 93
or something stupid like that?