The engine was designed to run the el-cheapo Kubota oil (which is probably Rotella or Delo). Stuff is dirt cheap compared to Amsoil. Over all of the years I've been screwing with engines, I've always found those high dollar lubricants to be wastes of money. They all do the same things. Lubricate, cool, prevent corrosion, and clean.
But all that is an arguement that y'all can hash out amongst yourselves. Not a single one of them has proven (at least not to me) to be worth the extra cost, at least not in my opinion, meaning, I've never heard one person say that their tractor engine lasted 10,000 hours longer than the exact same tractor that was used the exact same way at the exact same time and exact same conditions, then maintained exactly the same; and the reason is because every single one of them-cars, trucks, tractors, mowers, anything that uses engine oil, is used differently. Therefore, I feel that the data has always been biased. $10/quart and 8 quarts you're at $80 for an oil change. Kubota 15w40 is sold at the local dealer for $4.35/quart. At 8 quarts, you're looking at $34.80. Under half the cost. You keep, maintain, and use the tractor for 1000 hours, that's 5 oil changes (every 200 hours) and the cost savings of, what, a little over $225 (otoh). $225 that could be used to pay the tractor off earlier, or fix the hood when a tree decides to run out in front of it, or whatever.