I did the Tranny Flush at the dealer soon after I bought it a year ago - they flushed it twice as it was very dark in color.Lots of choices, thankfully. Mazda Genuine ATF M-III or M-V is what you'll see listed first in your OM most likely. But aftermarket products like Valvoline MaxLife multi-vehicle ATF (red bottle), Castrol Transmax multi-vehicle ATF (black bottle) and others will be fine in your P/S system. So if you changed ATF in your tranny recently (assuming your 5 has A/T), you hopefully have some left - can use that as PSF.
Oops...deleted my own post. Sorry, user error.
One quart is more than enough if you're just topping off. To flush and get to nice clean fluid, you'll probably need a little more than a quart.