Other than flushing the system repeatedly you cant get all the contaminated oil out. Best you can do is run it to get it all hot, then retract all cylinders so the high end has the cylinder retracted to the max, lower the 3 PH and drain the oil, change the filter and refill. This should get most of the oil out. If you wanted get the remaining oil out disconnect all the hoses and drain them and the lower end of the cylinders so you have a practically empty cylinder and the hoses are empty and that would get almost all of it out. Raise the 3 PH lift by hand to cycle that cylinder also while draining.
Personally, I wouldnt go to that much trouble, just lower everthing down, dump the oil and put a couple gallons in cycle it several times then drain again, replace the filter and recharge it to the full mark with oil. Anything left at that point would be minor.
Just make sure you put in enough oil so you arent cavitating the pump and hydraulics work smoothly.