The ZT 48 XL has been offered with several different engines. More info needed. Kawasaki engine owners manuals are only a 10 second Google search away. I imagine Kohler and Briggs are too.
FR651V, FR691V, and FR730V all take 1.9 qts without filter change and 2.2 qts with it.
Good luck on the hydro oil recommendation. Hydro Gear says to use their kit (PART NO. 72750) which includes filters. Folks that do their own put in different stuff such as Rotella 15w40 conventional and Mobil 1 15w50 synthetic, which is what I use.
Gravely says 15w50 synthetic.
https://support.gravely.com/article/?id=62028198-ca7f-e711-80f4-c4346bad526c
