Hydraulic - stay with Kubota brand. Others can cause problems…
Air - There is an inner and an outer filter, but not all tractors came with the inner.
Add it if missing. Cost difference is minor between Kubota and Rock Auto, less than $5…
Motor Oil - Use a name brand full synthetic 5W-40 or 0W-40 diesel rated
(CJ-4 or CK-4 for newer tractors with emissions). Rotella T6 is a favorite…
Oil Filters - HH164-32430 is Autozone S7328 and Wix 51307.
But the base is the same as Wix 51515 / Fram Ph8a / Motorcraft FL1a…
Much longer filter…
Try Rock Auto for good prices…
I snagged some Champion ones for less than a dollar at Rock
when Fram bought out Champion a few years ago..…
A Wix 51773 supersize also fits, but may be too long, and get caught or snagged..