I just purchased a 1961 2010 diesel and had the same experience. The drain plug is I believe a 1 1/8 bolt on the bottom of tractor directly being the hydraulic filter. The fill bolt is also the same size below the seat on the right side of tractor. I drained the fluid, filled the Transmission with diesel (per JD mechanic) and rocked ( by hand) the tractor back and forth and drained, two times. My tractor had a screen type hydraulic filter which I cleaned and then filled with fresh hydraulic oil. I plan to operate for a few hours and drain fluid and replace.