The 4610 is of the series of my 3910 and the manual says use 134D, also goes by J20C (a John Deer spec that usually shows up if any spec shows), the same fluid you put in your tranny and your main hydraulic/differential sump.
Under the cover of the pump, at the rear, one bolt holds it on, is an oring and filter. ebay has them if nobody else does....just troll for Ford 4610 Power Steering Filter. About 9 bucks with free shipping if you get the right seller. Have it in 3-4 days.
Full is even with the bottom of the hole....if you put any more in it would run out. On mine Ineed a pair of pliers to turn the cap the last ⅛" that completes the seal...lot of resistance but deliberately made that way to keep it from falling off and leaking.
Tractor Supply "Premium" Trans/hydraulic fluid in the black 5 gallon pail meets the specs for all your fluid needs except engine oil....read the back of the container.