Walmart has Shell Spirax, the Donax TD replacement. It is recommended for the orange and red Daedong tractors, McCormick, Kioti, etc. I don't know if it's ok for the Daedong tractors if they are painted white.
If you ever hook up your hydraulics to a cylinder or implement that another tractor hooked up, you will mix more that will be left in a change. In my opinion, you are worrying too much.