2008 Kia Rio misfiring

Have a 2008 kia rio. Ran low on gas last week. I added about a gallon and a half from a gas can that has been sitting in garage since late october early november. Next morning car running rough and check engine light on. I drove to gas station and filled up on 89 octane and put in STP gas treatment. Still ran rough and still check engine. Took to local parts store and ran codes. Misfiring on all four cylinders. Took to my mechanic. He changed plugs and put in another fuel additive. Said to run down the current tank of gas and see what happens. Ran it 150 miles today at various speeds up to 80 miles an hour. No change. Still have check engine light and running rough. Also appears to have less power than it did when it was running correctly. Just turned 100,000 miles and has no major mechanical work. Also has original timing belt.