Dot 3 or 4 (they can be mixed) The Dot 4 has a slightly higher boiling point but absorbs moisture 20% faster than DOT 3 which lowers the boiling point. I can't really say when (time wise) they become much the same in temperature. I Typically I flush all my cars and trucks brake fluid every two years but that's just me.
https://www.autozone.com/diy/brakes/...-4-brake-fluid
As far as your slave cylinder. Is it an external? Leaking? Internal type? post a pic of your setup. Typically there are equivalents available.