I disagree 100%. One of my M's has Chevron THF All weather Synthetic in the gearcase (14.5 gallons) and the other has Rotella HD in the gearcase (14.5 gallons). I did that to compare the 2 in real world application.
The Chevron is more expensive than the Kubota fluid and I've used it for years with no issues, in fact the flowability in sub freezing temps is excellent (I use it up here to move snow in the winter), however the Rotella HD also exhibits the same flowability in sub freezing temps and is a lot less expensive than the Chevron is (120 a pail versus 65 at TSC) and the Rotella HD is Kubota compatible (as listed on the pail).
So far, 2 years of farming and winter use, zero issues other than the Rotella is clear (like Kubota fluid) whereas the Chevron is orange so you can readily see the fluid level on the dipstick or sight glass, whichever you have.
If the tractor is a hydrostat, then I'd only use Kubota fluid, but a gear drive, don't matter. The only thing impacted on a gear drive unit as far as use would be the hydraulics because the hydraulics share the same fluid as the gearbox.
In a nutshell I disagree with your comment 100% (again, if it's a hydrostat I agree< but if it isn't, then I disagree.