^ Oil light came on? That's never a good sign.
Someone didn't put in enough oil last change, it's leaking, it's been far too long since the last change or it's burning oil. What ever it is, (hopefully not burning oil) thisit could have a lot to do with you poor mileage.
I still get 450/tank driving around the city and I avg 580/tank with 50/50 city/hwy driving.
http://www.fuelly.com/driver/pmccartney/g37
When no one is around or looking, I drive like I'm stealing it.