I'm due for to change the hydraulic fluid/filter in my 1723E. I have researched the forums to see if substituting a different hydraulic oil instead of the the Permatran 821XL is acceptable. I stopped by my dealer and spoke to the service manager asking for the appropriate fluid. He said they use ARGuard Premium Universal Fluid from the American Refining Group. He also said it meets all specs for use in the GC1723E. He also went on to say that "We use this in all our equipment" (they sell Massey, Kioti and several other lawn care brands, plus rent heavy equipment).
The back label of the container notes it is recommended for Agco-Allis Power Fluid 821XL (and several other brands hydraulic fluid), plus it was $74 for 5 gallons a significant savings over the Permatran. If anyone is looking for a substitute for the Permatran fluid, this one may be one to look at.