Yes, you should be using a product called Power Service in the white bottle or any of the other antil-gelling compounds. Power Service is one of the most used products by the Kubota owners that I know. You can purchase it at Walmart or buy a case of bottles inexpensively from Amazon. Assuming you are going to have your tractor for a very long time, just buy the case and save some money. It doesn't go bad, but it does get more expensive every year, just like Kubota parts. Last year I think it was $75 a case.
Power Service 1080-06-6PK +Cetane Boost Diesel Fuel Supplement Anti-Freezer - 80 oz., (Pack of 6) $102.87