Hi,

My $0.02.  Use ATF for a 'flush'.  Let the oil get low, to bottom
marker, just prior to oil change and bung in a quart of ATF.  Drive
gently in town for a couple of days, then drain and use new OEM filter
and new oil.
Bardahl makes a hydraulic valve lifter additive that will flush your
engine very fast, may lift chunks of crud and float them into somewhere
small.  ATF loosens crud more gently, it's meant to keep the valve body
in the autotrans working well.  I've used this to get rid of pesky valve
noise after sitting for a long time (winter storage or DPO).  Works
every time.

I do this once a year as preventative maintenance. Do about 3 other
regular oil/filter changes also. Fresh oil every 3K miles is the best
flush, especially for short haul driving and winter, when water and low
oil temps create a lot of cr*p in the oil.  I've heard that some oils
will dump paraffin (wax) out, leaving a slimey goo in the block.  Quaker
State was mentioned.  Can't comment on this, I use Castrol, which leaves
a fine brown sludge in the bottom of each bottle!?  bye, Tim