Do spark plugs have to be iridium… or do they go normal?

Hi, I play with the idea to screw new spark plugs into the smart. Lt.Smart are the ones in my Smart 451 only at 90000 km. Has the car run now 39000km Do I have to screw in the expensive Iridium candles? Or do the “normal” spark plugs work too? Up to corresponding FIN were only “normal” spark plugs prescribed…. after/from corresponding fin the interval and the candle type from Normal to Iridium was changed. Greetings