You will have to quantify the amount that drains out with your method. BTW leaving flushing compound in the motor is going to cause problems as it is corrosive - thats how it works. I never use them or oil flush liquids. If its blocked (radiator, heater matrix etc) take it out anf fix it properly..
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i