Depends. If it's readily available Kubota hydraulic fluid (not oil btw, fluid) is fine but in lieu of that, any Kubota compatible fluid will work, preferrably synthetic for cold weather operation (if you live in a cold climate.
Myself, I use Chevron All Weather Synthetic in my M's., because my jobber is a Chevron dealer and it's readily available. With me, that is key.
Just bought another M and I'm about to change fluid and engine oil and all the filters, including both hydraulic filters, air cleaner filters (there are 2, inner and outer), oil and fuel filters. Because it's used, assume the previous owner was lax o maintenance so start fresh and at square one, change everything.
My 2 cents for what it's worth. I also presume from the size of the tractor it's not a 'hobby' tractor so just bite the bullet and do it. Grease it too if it has a loader and grease the axle pivot and the 3 point linkage.