My 2003 X350 4,2 na 350.oookm/218k miles got an engine flush recently just before winter conditions .
Warm engine ,liqui moly engine flush ,12 min running idle , then drain for half an hour, pored a glass of fresh oil in just to rinse out some more of the residues of the flush.
Well it did not harm but it was not really worth it . Those engines stay superclean inside , oil color changes to darker only after about 6oookm 4ooo miles . I drove 25.000km in 2 years did one initial oilchange when I got the kitten december 2020 after doing 600 km with the old oil in it with 326k the again at 338k and now 351k
the drained oil did not have any sludge ,gunk,steele or aluminum debris on the bottom of the container .
they are nearly bullet proof . Just two days ago I hammerd it 3 times from rolling start to 110km/h runs perfect

218k miles on your 4.2? That's awesome to hear. Have heard nothing but good things about these engines which was one of the reasons I decided to get one. Even with previous high mileage cars (mostly GM) have never bothered to "flush" the engine. I always felt that keeping on top of the oil changes and other maintenance was enough to keep it in good running order.