I've been looking into this since my 2500 will be getting refilled pretty soon. Looks to me like my best solution is standard 303 tractor hydraulic fluid. It can be found a lot of places and it's pretty cheap (often less than $30 for 5 gal.) Basically if you are running a hydrostat drive and hydraulic pump to the arms & such, it's hard to beat yer basic 303 tractor hydraulic fluid, especially for the price. I would pay more for something else if would be better for it, but that's pretty much what you want. link