[509Doughboy]

Hello, all I am from Eastern Washington state. I have a 1992 Cub Cadet 1330. I bought this machine new. It has been a phenomenal tractor over the years. I’m in the middle of some winter time, maintenance new PTO and bearings and belts on the deck. Can anyone help me out with the hydrostat transmission. It still seems to work flawlessly. I add hydraulic oil to the reservoir periodically keeping the oil about a half inch from the top..
Should I be thinking about somehow draining it and replacing the oil?
Thank you so much. Steve