[SAJ]

KenBolton, I have an image in my head of a motor with sludge containing years of unfiltered metal particles from normal wear processes, plus cotton fibres released from a worn cam gear and maybe gritty bits where the dipstick missed the hole on first try by a half blind pump jockey years ago when garages checked your oil. Also bits of babbit and other particulates all locked into the unfiltered goop that set and sits in valve chamber and sump.
In my image, fuel oil might unlock this detritus and diesel or kerosene certainly would. And it would pass right through the oil pump and into the gravity-fed bearings, and be deposited in the tray where the conrod scoops deliver it into the big ends.
A clean engine wouldn't suffer this ( and wouldn't need flushing anyway) but an old, dirty, unfiltered one would not like it at all.
I may be wrong here, but I am a great fan of removing sump and valve cover, cleaning everything out properly, having first blocked the mains oil tubes with pipe cleaners if really dirty in the valve chamber. And then using full synthetic oil with a good amount of zinc and phosphorous (1200 parts per million or thereabouts), added at the refinery not by the car owner, because too much ZDDP is corrosive.
I use Mobil 1 15W-50 in temperate Auckland with full flow oil and air filters.
Before anyone comments that Model A's have steel camshafts, not cast iron, with light valve spring loads, the ZDDP is in my engines to protect pistons, bores and bearings more than the cams and tappets, which we know are lightly loaded in the standard Model A engine.
As I said, just my opinion, but based on a study of tribology during university years and manufacturing industrial oils and greases for many years after I qualified.
SAJ in NZ