The gunk is there because you didn't change the oil in time ! No need to do any flushes when you use the right oil and change it in time. Oil filters and drain plugs don't need to be torque tightened, hand tighten, then a little more with a wrench, that's enough, has worked for over 20 years with multiple cars.
What's the name of that tool you inserted on the oil samp, the small one?
The gunk is there because you didn't change the oil in time !
No need to do any flushes when you use the right oil and change it in time.
Oil filters and drain plugs don't need to be torque tightened, hand tighten, then a little more with a wrench, that's enough, has worked for over 20 years with multiple cars.
I don't think he had the car from new so it would be the previous owner that left the oil in too long.