1995 Hyundai Accent Engine/ AC PROBLEMS

My car runs very rough and it's MPG is really poor. Once the gas goes below half, the car begins to jerk and sputter as if going out of gas and sometimes stops completely as if there is no power. When I fill the tank up it stops but consumes the gas within a short while and runs really rough while idling. I have changed the plugs and the wires. But the problem still persists.

The oil also magically disappear. There is no leak in my car but it just goes dry after a while without the indicator coming on

And once I switch on the AC the car dies or it cannot accelerate as much as before. I will appreciate any diagnosis I can get.