Findings
Nicker, I have been researching your fluid compatibility question and have determined that you should have no problems with compatibility.
The ASV spec oil is Chevron Rykon MV, which is a premium all-season multi-viscosity hydraulic fluid. The JD20 fluids are basically the same specification and are also multi-viscosity and available is various viscosities.
It's your machine, but I see no reason to drain the system. I would recommend oil analysis on the hydraulic fluid and engine oil to determine when a system drain is necessary vs. changing on hours. Oil analysis is a valuable tool that you can use to see what is going on inside the machine. The fluids are the life-blood of the machine, and oil analysis is a tool that technician use on equipment, just like a doctor would a blood-sample to determine the health of a patient.
Most oil samples run from $10.00 for the very basic, to $30.00 for a complete analysis of the fluid. If you need help finding a lab, I can send you the addresses and contact information for a couple that I use. Again, I can stress enough how valuable oil analysis is from a reliability and economical standpoint. Saving you money and reducing equipment down time.
Good luck and let me know if I can assist you with anything else.