1. I agree with Purple. Re: synthetic - It's a better oil for less engine wear and better mileage.
2. Agree w/Purple.
3. Agree w/Purple. Disagree w/you. Your getting better mileage than most of us. (18.50 MPG for me.
) Try more air pressure in tires for better MPG.
4. Agree w/purple. Your GT shouldn't stall. Have dealer check it out.