Climate here varied from 88F-20F during that drain interval, including extreme duty cycles. (I basically raced the car).
It routinely gets to 100F+ in Maryland during the summer...I wouldn't call it cool by any means :).
Amsoil synthetic is very good. If you do use it, please considering doing a UOA on your second change with it. I'm curious as to the results, as I'm sure many others are.
Dealers are using Motorcraft 5w-20 Semi-Synthetic...if their following factory spec. Its already been shown though, that some dealers put in whatever crude oil they have laying around (usually 10w-30) with a poor grade fram filter.
Stock filter (good quality) is a Motorcraft FL-820S (On the V6).
on professional opinions on engine breakin.
Many switch to synthetic at their second oil change (which is usually 500-1000 miles on the engine).
If anything, synthetic would allow for a more "gradual" wear in after the inital is done, smoothing the engine out.
But don't listen to me, read for yourself and do some research.
The sooner you switch to synthetic (after making sure the engine/drivetrain is fine..aka manufacturer break-in by most standards) the sooner you get the added protection.
For transaxle fluid changes, its usually around 5k miles to switch.