gt 245 transmission service

in the manual it says transmission is divided into two internally connected cavities with two different drain plugs, one being for the differential gear side and the other for the hydrostatic side- then it says -do not remove the sealed black cap "c" and to fill only at the reservoir bottle--------is this correct ------ is there internal ports which allow oil to fill both tanks from one fill source-------should I run tractor to get oil hot prior to changing so any debris would flush out when the drain plugs are removed= again ---------------thanks for any help------------------Jimbo