My opinion will most likely be contrary to most others, but Kubota Super UDT isn't like motor oil, and I don't believe that it wears out. A friend that is a Bobcat tech says that they don't recommend changing the hydrostatic oil unless there is a sign of contamination from condensate water, otherwise, it is thousands of hours. I know that replacing a hydrostat is expensive, but apparently, yours has served you well for many 100s of hours. I don't know if you can have an oil test on hydraulic fluids like engine oils, but if that is possible, that might be a good start. On the other hand, the cost of the oil and filter isn't going to put you in the poor house, so if you will sleep better at night, then do the full service.