Yes, diffent gass WILL affect it, im in canada and when i go into US and get US gas, my truck runs like shit and bad gasmilage but, its also cheaper then canadian gas.
cheak plugs, i bet the electrode on them is all gone unlesd youve changed them previosly
fuel filter wont hurt gas milage, will just run like shit from not enough fuel
pu a cold air intake and an exhaust on, will get batter gas milage and some performance, its about making your engine breath better