uh guys.
This mustang does not take power steering fluid. It takes mercon transmission fluid in the power steering system.
Check the owners manual…
I recommend you flush the system (turkey baster method). You can buy a gallon of supertech walmart trans fluid for like 7 bucks. You need about a gallon of fluid to flush it through and get it clean.
Suck the fluid out with the turkey baster, fill with fresh fluid, put the cap back on, start the car, crank the wheel back and forth to activate the pump. Shut the car off, suck the fluid out and repeat.
If you get all that black shitty fluid out of it, the power steering pump will not whine (assuming its not damaged).
If you were keeping the car, I’d say to use mercon V semi synthetic fluid, but for your purposes, mercon is fine.