my 2006 altis auto, that i have had for over 6 years has always done 15-16km/l on the motorway and 8.5 in stop start traffic driving in city and overall 10km/l in normal city driving on main roads. Ac full time on and it makes no difference anyway on these 1.8 engines on fuel eco and i dont baby the car and instead make good use of the 1.8. no need to mess with plugs or catalytic convertor, use original iridum plugs mentioned in the owners manual and they should be of the correct heat range.
i use fully synthetic oil of 0W-40 viscosity, using lower or higher viscosity than this can also affect fuel economy for better or worse but i feel this grade offers the best protection.