Dad used a quart of kerosene in the engine oil to flush gunk just before an oil change. Dump it in a warm engine, start the engine and let it run for a few minutes, then shut it down and let it drain back into the pan, without ever tou thing the throttle. Idle only. Then drain an refill with fresh oil.Basically the same but we always used kerosene and atf and had good results.
Worked well for dirty engine. His dad had a used car lot. Dad bought a lot of junkers over the years, and ran them to death, but did feed them good gas, and change the oil regularly.