I have kept the hydraulic/differential oil changed every few years in my old 1957 to-35 so it was not in too bad of shape.
I just get the tractor gear train as hot as I can plowing, that is about the hardest work it ever does. Then drive up to shop and drained out the oil, let it drain overnight. I just add the gl-1 90 weight oil, 8 gals and was good to go. Since the system was working good, I was concerned about adding any special cleaner that might affect the operation.
In winter months the gl-1 oil makes the lift slow, at the next oil change I might go with 10w40 oil, the new oils should protect the yellow metal ok at this point in the old tractors life.
I get my gl-1 oil at Napa.
Since the system has no oil filter the gl-1 oil is designed to let the dirt particles drop out to the bottom of the sump, the newer oils are designed to hold the dirt particles suspended so the oil filter will take them out.