With further reading, I conclude that flushes work for most people, but an unlucky few suffer major engine failure. One person even claimed it can happen with Marvel Mystery Oil in the crankcase. Very unlucky I would say.
One mechanic made interesting remarks about flushes, and MMO:
www.quora.com/How-would-you-rate-Marvel-Mystery-Oil-as-a-crankcase-additive
The takeaway for me is, be cautious what I undertake, and careful how I do it.
Semi trucks go 25,000 miles between oil changes. To make a profit, truckers stay on the road and out of the shop. If they can do it, why not I. They're pros, they know. Car mechanics, OTOH, don't want any accusation of liability from customers, so they keep preaching the 3,000 mile oil change religion.
www.internationalusedtrucks.com/maintenance-tips/semi-truck-oil-change-intervals/