I haven't read much about it with regard to the Sienna, but my last new car was a 2006 CR-V for which you were explicitly told to absolutely not change the oil early because the factory fill was a special high-moly oil that was supposed to help with engine break-in (allegedly it wasn't just straight Honda oil you'd get from the dealer). I'm fairly certain that people that change after 1000 miles are practicing major overkill (no, not trying to start a fight - your car, do whatever the heck you want).
I did read in my manual that for the first 1000 miles you are to avoid quick acceleration, hard braking and constant speeds. If you are a cruise user (as I am), I would suggest that on your 300 mile journey you should just change the speed by 5-10 miles per hour every 5 or 10 minutes.