Hi Eric,
I just bought a Gravely ZT52 HD last April. At the end of the season last fall, I got a message on the timer saying it was time to change the hydraulic fluid. I was just about done with it for the year so I put it up and decided to do it in the spring. Well spring is here. I looked in the manual and my machine uses 20W50 motor oil. I have the printed manual, but I downloaded one and saved it to my computer in case I ever lose track of the printed one. Here's a link where you should be able to get a manual for your machine. You just need the model and serial numbers from the machine and you'll be all set.
Ariens Order Owners Manuals
Good luck!