Plenty of garages and dealerships use flush Zeke. I assume that they would know what they are doing. My brother in law is a mech in Adelaide and uses it all the time and hes been in the industry 40 years or more.
I Have pulled a mates engine down that had did 20.000km and the sump was clean as, spot on clean you could eat off it and that was back in 1984 I remember my mate telling his elder brother this fact, and big bros said B/S ! as he did not believe it at all.
The engine was a Holden 202 that was worked and when like a rocket and was taken to 7000RPM and flogged hard, what happened was that the block cracked across the water jackets as they do when they make such power, so we had to pull it out and get another block.
Old mate changed the oil ever 5000km.
I am sure they do use oil flush as you never know with engines of today, as some fools do not service them. I would not go past 10.000km and 12 months regardless of even syn oil because more than that your just looking for trouble, 15,000km if on mainly highway driving would be ok.
Hey have any of you lot used Bitron called Xsnano now claims of diesel power up to 34% and better fuel and reduces emission up to 98%