Most of those same vehicles you refer to also spec 5w-30 in other countries, so .... you logic is somewhat flawed. That recommendation of syns is based on a desire to meet an EPA requirement in North America, and isn't based on what's "needed" by the engine by any means. Syns are not being recommended for cars today because it's "better" for the engine; those recommendations are because they are attempting to meet both a fuel economy requirement, and to a lessor degree also work towards longer OCIs in an attempt to become more eco-friendly (fewer oil changes).
So you're saying the OP should base his lube selection on today's economy and ecology concepts for cars, and then apply that to a tractor from nearly two decades ago?
Here's the way I see the topic, using logic rather than emotion as a driver for decisions:
1) I agree that using a syn (5w-40) won't hurt the tractor engine at all
2) using that same syn (5w-40) isn't oging to help it at all, either
3) Kubota engines are incredibly robust and are not known for needing syns to ward off problems, because generally there are not any problems to be concerned with in the first place
4) syn oils have an advantage at cold weather starts; but I mean uber-stupid cold temps, like those below zero deg F. This is because they aide in less parasitic draw during the pumpability of during starts. This ambient temp issue is NOT an problem in TN where the OP is at, so it's a "benefit" he will NEVER actually capitalize on. If one believes a thinner oil is needed for cold starts, just use a 10w-30 instead of a 15w-40.
5) syn oils (group IV and V) are a good consideration if one is going to extend OCIs out past the recommended duration; there's no mention by the OP he wants to do that
6) there is plenty of UOA proof that in "normal" OCIs, syns don't reduce any wear trends over the conventional oil choices; none whatsoever
So, maybe, actually you're "underthinking" this.
There is one thing the syn lube will be better at in the OPs situation ... wasting his money for no tangible benefit.
If that's the goal, then by all means keep banging that "always use syns" drum.
