Yeah it's tempting to go to TSC and get 5 gallons for $35, it says it will work right on the bucket. Will it work, yes it will. The real question is will your tractor perform as it should or have the same level of pertection? That's a big fat NO. The hydraulics will run hot in the summer, whine, chatter and jerk. In the winter everything suffers, steering is slow and stiff, same with the loader and 3pt.
I see you have a M8540 listed so that's at least a $40K + investment. My question to you is this, 10 or 15 yrs from now do you want a piece of scrap metal or do you want a fully functional tractor that works as it should? That's the difference between cheap fluid and quality fluid, protect your investment. JMHO