Looking at your location I would only use the Super UDT2 because of its ability to flow freely in extremely cold climates. I believe that this is the reason that the original factory fill is with the Super UDT oil since Kubota doesn't know exactly where the tractor is going to see service. Think of it this way, if you use the Super UDT2 and it takes about a gallon to fill the front axle, it will cost you about $20-25. If you fill it with 80-90W gear lube it will cost you about $10. If the front axle doesn't get proper lubrication having it repaired is going to be a whole lot more than the $10 or $15 savings. Now, think about which lubricant would be the least expensive for Kubota to install at the factory. They could save a lot of money by using gear oil over the Super UDT2, but they don't. Now ask yourself why, and the answer is quite simple. They have to guarantee the tractor for x amount of years, and front axle failures would cost them a lot of money, way more than they would have saved by using the lesser expensive gear lube. If you lived in the South, the gear lube would be fine because it would flow more freely in the warmer climate. Your tractor, it's your choice, but keep in mind that you will ultimately be bearing the expense of repairs if you make the wrong choice.