Glad to hear you like the tractor. Still think what you paid has to make it a good deal. What I did with the filters on mine was just get the # off the filters that were on the tractor, Wix has a website that crosses over from just about anything. If that is not possible I would call the dealer.
As for oil I use 15/40 mainly because that's what I have on the shelf for my other trucks. If my tractor sat outside in the winter I'm sure it would need something lighter. I would think that tans/hydraulic fluid for the tranny, back diff, hydraulics probably 1 tank for them all. Mine has gear oil in the front diff but with a Hydro it might also be lubricated from the same system. I would call the dealer and get his suggestions and then talk to your fuel/oil distributor and get his opinion for local conditions. If it is going to be outside in the winter you might have to go with different oils in the summer and winter, or find a compromise of some kind. One time with mine I left it outside in the fall and before I got around to putting it in the heated shop it got to -30. I plugged it in and it started OK but it took a couple minutes for the hydraulics to raise the loader enough to move the tractor, everything I could do to turn the steering wheel and she made some god offal noises driving ahead 20 ft and backing 20 ft back into the shop. Something you don't want to do every time. After it warmed up everything was fine. Now in the winter it stays inside, goes outside for a couple hrs pushing snow and then back inside, and I run heavy oil all year round. I would guess your Hydro would be more prone to cold weather problems but nothing that can't be overcome.
On another note, how does your tractor handle the 6 mower? I have a 5 ft rough cut that I have too much power for in light grass, I've been toying with the Idea of a 6 or 7 ft finishing but I just don't know how much extra it would be.
Gary