Check the MOT history
has it failed on emissions in the past ?
how is the engine now you changed the oil ? any noisy valve lifters ? knocks etc ? smoke when the engine is up to temperature ? is this the 120k miles one ?
5w30 fully synthetic is thin and will flush sludge from all the engine IF it is there (if engine has done short journeys,not had regular oil changes,had thick oil in the past)
I've had this in the past ,a 1.6 with only 80k that a garage put flushing oil in to reduce emissions then fully synthetic 5w30 (it did not work, just made it worse ) all the sludge just stuck in the oil control rings ,jamming them in the pistons
it smoked like a steam train when warm, I got the car,stripped it ,new rings ,cleaned sump etc and found the emissions fail was down to a clogged injector
your car may have had thick oil in or not been changed enough(very likely from what you say) ,
hopefully a couple of oil and filter changes will resolve it .