My bet is that Honda said use 10w30 because it will move a tiny bit more freely, and give ever so slightly better fuel mileage.
Tiny bit of mileage X 1,000,000 cars = big tax break.
Just like the 10w30 engine oil imo.
My other bet, is that your third gear synchro is worn, and after the long highway speed trip the oil has heated up and thinned out, and is no longer thick enough for the synchro clutch doodad to work.
When mine is COLD, it's slightly harder to shift then normal (and it will NOT go into first if your moving, just flat out won't go), but once it's been driven for a couple minutes it shifts great.
Still shifts great after an hour of 80-90mph I5 driving, too
From what i've heard, Dano is dead on with the synthetics bit, as usual.
I mainly use gear oil because it's what we have at the shop, in the tranny oil pump.
10w30 would involve a long, slow funnel
Plus i like thicker oil all the way around, more protection imo.
I run 80w90 gear oil in my tr7's tranny too, despite it calling for ATF.
I can FEEL the difference in weight causing some drag, but it shifts a lot more smoothly. (The TR7 tranny is weird, it has an input shaft driven oil pump that lubes things, like an auto tranny, so it's probably having to pump the system up to a couple hundred more pounds then it ought to, if i let the clutch out in neutral it drops a solid 200 rpm hot, and 400 cold)