Deutz Allis hydrostatic

I have an old Duetz Allis 612H thar I replaced the Eaton model 7 hydrostatic transmission with a used one. It's doing fine a bit noisier than the original but smooth. The fluid is a little dark and I'd like to change it. I'm not sure where to drain it or where to refill it. I'm told I can use 20 weight oil for the refill. Anyone have any info?