It's really a large system.
If you measure the amount you drain, you will discover you are well short of getting all the fluid out.
Which is why I follow the instructions and flush with clean water, several times. Get every last bit of the old stuff out. Which is even more important if you are switching from one type (standard) to another (universal-long-life).
When you are done you might find you can -just- get 15 quarts of straight fluid in (ie DON'T buy pre-mixed)
As I recall Ford does NOT recommend Long-Life fluid in that engine, I would use either Standard or Universal-Long Life.