The specifications are at the back of the O&M manual. Mine was pg 55, but my tractor was a 1988 1720. It said,
Rear Axle and Transmision 28.5 qts (~ = 7.13 gal)
Power Steering 1.9 qts
Front Axle Differential Case and Final (outer drive) Reduction 5.3 qts
New fluids often are multiviscosity, like 10w-30. For a tractor such as this, I like the TSC Universal Tractor Hydraulic fluid, which sometimes in the 5 gal size goes on sale, and it is multi-vis.
Don't forget the filters and note that there are multiple drain points on both the rear, tranny and front axle/final drive.