Hello Welcome to JA!
I would stick with the – Hy-Gard hydraulic/transmission oil
Is there a reasion you want to change to a diffrent fluid?
– Hy-Gard oil has a viscosity between the ISO grades of 46 and 68 @ 40°C.– Exceeds JDM J2OC specification. Some competitive oils may meet these minimum J20 specifications, but none meet the performance requirements placed on Hy-Gard oil
The rest of your post on your question just showed on my end Yes i would run the -HY guard in the whole tractor
– Hy-Gard hydraulic/transmission oil provides the best performance and wear protection for John Deere hydraulic and transmission systems.– Recommended for equipment with either common system reservoirs or separate transmission and hydraulic systems, including those with wet brakes and wet clutches.
Stick with the – Hy-Gard hydraulic/transmission oil You will save a lot of wear on parts
Let me know if i can help you more! RD
The capacity of the system is 10 gallons. Should I drain the oil and replace it with hy-guard then?
Yes that will be the way to go