[AlbinoMonkeyRat]

perhaps the type of oil is what's hurting it. usually, once a car is "used to" a certain kind of oil, it operates more efficiently on that type. However, that shouldn't account for a huge change in the mileage.

There are also other things you can do:
-fuel system cleaner (the type you pour into the gas)
-tune up
-clean the butterfly valve in the throttle body. if it gets too dirty, the valve flaps will stick open or closed. if they stick open, you will run lean, if they stick closed, you'll choke it (no air). (if you have a CAI or a WAI, you already know where this is...if you don't, it will be a bitch to get to...)

also (joke) maybe they switched to winter blend gas already in Canada...