643 hydraulic oil

[Shortbed70]

I have a 85 643 3 cylinder deisel and the hydro oil seems pretty dark. I just did a service on it earlier last year but haven't use it much maybe 30 hrs! What can I do to drain the oil completly and refill or will it hurt anything? It runs fine but the couplers for attachments leak and I'm not sure it might have a motor leaking into crankcase but might be very minor.