Most of these units will have HydroGear, Kanzaki, or Parker pumps and similar wheel motors. The newer hydro units specify 10-50 Mobil 1 synthetic oil or similar.
Check the pump type and model # and then that Mfg. web site for suggested oils. Most of these units have a resivoir with a cap and usually the oil type is on that tank or cap.
One more thing, in order to get all the oil out to do a full replacement you may need to drain the pump/motors too, then getting oil back through all these lines you should leave the lowest point(s) open while adding oil then tightening that connection. You want to make sure you get all the lines refilled before running the pumps. (you could pull the plugs and use the starter to charge the pumps before running full tilt as one option).
If you dont like the sounds of this procedure then have a dealer do it - may be $ well spent.