My neighbor was selling an extra b7800 he had that had a problem, sitting outside for 8 years, I brought over a new battery and 5 gallon of tractor supply Travelers hyd/trans fluid filled it up from mt and fired it up and drove it to my house and been using as is for the last 2.5 years. That's the way I am rolling and it kept my old Ford tractor going for over 25 years. I will get the haters but time will tell, I wont be here in another 25 years.
If you have the HST transmission UDT is strongly recommended. If you have any other transmission I suspect any good grade of hydraulic oil would be fine.
I just changed the oil in my BX24. I am somewhat superstitious so I went with the Kubota brand UDT. Total capacity is a little over 3 gallons. I bought a 2.5 gallon jug and the had to go back for a gallon (just to be sure) Total bill was around $110. For me that is cheap insurance. I just don't want to risk it to save a few dollars.
As a side note, I read the manual and knew the capacity was listed as 3.1 gallons. I had a little left in a gallon jug. So I thought with the 2.5 gallon jug, the leftover in my other jug and the fluid left in the lines and cylinders, I would have enough. That brought the level up to the very bottom of the range on the dipstick. (DOH!) Hence, I had to get another gallon to top off. In reality a quart probably would have been enough, but I didn't want to make another 20 mile trip to the dealer.
Cat D3, Deere 110 TLB, Kubota BX23 and L3800 and RTV900 with restored 1948 Deere M, 1949 Farmall Cub, 1953 Ford Jubliee and 1957 Ford 740 Row Crop, Craftsman Mower, Deere 350C Dozer 50 assorted vehicles from 1905 to 2006