My opinion is that if you need a motor flush because of crud buildup, a motor flush can do more damage than good. loosening the stuff quick can then plug other passages and the oil filter.
You'd be better off doing a few short oil changes with a high quality synthetic oil with lots of detergents to slowly release and hold anything.
but if you've done good maintenance, current oils will keep the inside of an engine and it's passages very clean. maybe a slight thin varnish film, but nothing else inside.