Hello, car due for MOT next week, just had the imissions set?

it was 6.2. It is now just under 3, that is fine. With the car running rich, my exhuste will be full of soot, right. Should i put some cleanner in to tank , just to make sure? My question is this, are all cleanners the same, i have some car lube. But many say Forte is the best, pls help!!!!