Uneventful MTF change today at the dealer with the correct LT-5 fluid. I had been noticing that the shift action was a bit notchier than when new, but it may also just been my imagination. After the fluid change, the shift action is DEFINITELY improved back to new -- like butter.
It makes me question if factory-spec'd fluid change intervals are too long in duration -- for our US cars tranny fluid technically shouldn't be changed until the 3rd oil change. If there was a noticeable improvement in shift feel with a fluid change at 1400 miles, what would have been going on in the gearbox at 20,000 miles?
The answer is probably DIY in between manufacturer-specified intervals.