Oil flush should really be called a "sludge flush" ! Once oil has been used long past its serviceable life it starts to slow down, become guey, gummy and thick. Add heat to this and it will eventually become sludge, so thick that is next to impossible to get it off.
The only real way to get it off is like spraying oven cleaner inside an oven, let it sit for two hours then clean the oven. Motors in cars wont stand the equivalent of oven cleaner because the motor has to be running for it work. The only alternative is to start to take the motor apart and literally blast the caked-on, baked-on sludge from parts of the lNSIDE the motor! NO SUCH THING AS ENGINE FLUSH