Even in a sealed unit changing the hydro oil shouldn't be too difficult, just maybe a little time consuming as it's best to remove the transmission & invert to drain- no matter what brand the transmission is, there'll be a filler threaded "plug" or push "cap" that was used at the factory to initially fill the unit (& can also be used to drain the unit as a invariably a sealed unit doesn't unfortuntaely by design have a drain bolt/plug ) - as alternative to removing/inverting the unit you can use a hand powered oil suction gun/pump (usually found in auto or marine shops) to remove the oil via from the filler plug/cap however this method is as effective as it doesn't always remove all oil/gunge.
It's good maintenance practice to change hydro oil & in my JD LT155 which has now done close on 1100hrs ( the tranny performs as new) every 100hrs I replace the oil with premium synthetic product (the TuffTorque tranny supposedly is sealed for life too but the way I see it : oil breaks down, & all hydrostatics hate heat & degraded/dirty oil so it's cheap insurance & relatively easy to do on this so called sealed unit) & ditto for a sealed Peerless tranny I have in a homebuilt GT - the trouble is by the time you notice diminished performance in a hydro due to oil breakdown it's usually too late to recover the unit by an oil change.
When changing Hydro oil:-
- clean the unit thoroughly with compressed air or wash in water/dry prior to removing any filler plugs/caps, with hydro's any dirt ingress can quickly kill the unit
- the oil level is critical, so measure/mark & replace exactly how much you remove (there may be a fill level marked just inside the filler housing but it's not always there)
- there may be a small filter mesh screen & possbly a small spring just inside the filler housing which can be cleaned too is it drop out.
- take time to let the oil drain slowly to empty as much as possible
- also take time to fill with oil to allow the oil to distribute evenly in transmision & fill slowly to minimise any air bubbles/air locks forming in the oil, & also occassionally whilst filling rotate the axles slowly by hand to distribute the oil evenly in the hydro unit
- depending on the filler plug/cap fitment, once you replace firmly it may be prudent to use a small bead of silcon externally around plug/cap to exluded all possibility of dust ingress.
For hydro fluid I use Mobil 1 5w50 Engine oil.
Good luck & let us know how you get on