My tuppence worth:
My old 155 v6 beast has only ever had oil changes every 12,000 miles, using semi-synthetic 10W40.
When I had the rocker cover off circa 160k, to change a leaking rocker gasket I saw a lot of black gunge on the inside of the heads... varnish and stuff that looked like black chocolate that was impossible to shift. The oilways were a bit furred up.. but only very marginally.. about 5% of the diameter. Wear on the motor was negligible!
Way later.. when the head gasket split I took the heads off and cleaned all that poo out with kerosene. I believe the Wynn's flusher is mostly kerosene.. but anyway, the varnish and poo is kerosene soluble. I also tried all sorts of other stuff.. acetone, white spirit.. etc. Kerosene/Parafin is your man.
I refitted the heads and put a new oil and filter in there. I used semi-synth 10W40 "for diesels" as I reckoned, some bits might be loose and needed flushing away to the filter.. and other remaining stains, gunge, gum and crud only part-shifted or in areas I couldn't reach would be further cleaned up by the extra detergents in the "diesel" oil.
As expected, the oil went pretty black within 1250 miles.. which was okay as that coincided with a service interval (192,000) I just changed that oil for the same (for diesels) and ran that for the next stint of 12,000m. That oil stayed very clean for a long time (let's say "as normal".. so obviously the first change following the rebuild did clean up a lot of crud.
Peeking through the oil filler now shows still a veeeery clean engine. No gum or varnish at all (albeit just 12,000m since I cleaned the heads).
I did have an oil leak on the sump gasket for ages.. and that does not
appear to have got worse after using the "for diesel" oil (back on "petrol" now) so the "diesel" stuff doesn't knacker anything.. I guess as has been said, it's more gentle.
I did use engine flushers on that car a few times, pre-head gasket and while some blackness came out in the oil, it is obviously not THAT aggressive either as the heads were still black and gummy and horrible when I dismantled them for the head gasket.
a) Use a 6000 mile change interval... (less time for the oil to break down and clog up the engine).. or;
b) Use a synthetic oil (more resistant to breaking down and more detergents in it) or;
c) Use a "for diesel" oil (more detergents)... or;
d) Obviously, a "for diesel" oil but changed every 6000m. I doubt you need to "waste" an expensive synthetic oil by changing it at 6000m...
Good news is that my old beast has no wear on it's diet of "12,000 mile change intervals" and it has plodded on past 200k now. The "gunge" may be worse if you don't change the oil so that's the first thing to get right, whatever else you do (as above) to improve matters further.
For a manky engine.. use half a pint (~250ml) of kerosene, poured into the top of a hot engine... leave the beast fast idling at 2000rpm for 10 minutes and then change the oil and filter as normal. If it all leaks afterwards, then that's probably less serious than having that crud in the engine in the first place. After the kerosene/flush, use "for diesels" oil as a follow-up and change it after 6000m .. or sooner if it goes immediately black. You have to just judge it for yourself.