If you are going to drive on CNG please reduce the gap from 1.1 mm to 0.9 or from 0.43 to 0.39, also change the plugs twice as frequently as you would on petrol. You might consider using a colder spark plug such as BKR7E-11 but again manually shorten the gaps. factory set is 1.1.
If you can locate and afford iridium NGK IFR6J11 do not mess with the factory set gap, it will break.
DENSO K20PR-U11 is a recommended standard plug but "do numbar" are common.