Lots of things can cause poor fuel economy - have to do some diagnostic work, get a baseline of the car before you start throwing money at it. Can't really go by the gauge - as they're very linear in reading and have a relatively sluggish response. Have to fillup the tank (don't top off, just pump until it auto-shuts off), clear the trip odometer, drive normally - when it time to fillup, again - just fill the tank, don't top off. Divide the distance traveled by the gallons used. Do this over a couple of tanks, as you could have excessive idling one tank, have bad gas, different stations running different blends of gas, etc. which drop the mileage.
Most should be seeing a drop in fuel economy, as lots of stations have started rolling out the winterized gasoline. Refineries started in September, so most stations should be switched over, though some might have some old stock left if they don't see too much turn over.
The car consuming oil is more of a pressing matter. Need to find out exactly how much it is consuming - as that will influence fuel economy in the long run. As for possible causes, at that mileage and age - could be just about anything. From simple things like tire pressures and alignment or bad tank of gas - to ignition system, fuel system, induction system, cooling/general electrical, emissions, etc.