Newer excavators use HST drives, so I would definitely use a name-brand or certified
equivalent tractor-hydraulic fluid. I don't know if your machine has wet-clutch type
brakes. Don't use a plain hydraulic oil.
Anyway, I buy Chevron 1000 THF, Conoco 76 THF, and more recently Service Pro THF.
All have Kubota UDT and Deere J20C (as well as CAT, CASE, etc) listed as equivalents.
If you live near a big city, go to an oil distributor to save money. Walmart and NAPA
DO have certified oils in 5-gallon pails, however. Tractor dealers sometimes have
teaser discounts on oil, too.