I always put 3 quarts in, let it sit for a couple of minutes, then check the dipstick and go from there. I like to be precise. FYI, too much oil can make a mess of things.
Mazda says 3.7 or 3.8qts depending on where you read. Typically 4qts is a tick above full which won't hurt anything. To properly check the level start the car for a second after you fill the new oil then let it sit for a second and check the dipstick.
Everyone has their own opinion on oil. Mazda recommends 5W-30 on all their turbo engines including the FS-DET. I use Mobil1 full syn personally and a Mazda filter(only because i get them cheap).