There are folks that work for Ford on the site. They have addressed this myth every year for as long as I have been a member. If the search is working, you will turn up several past posts. This rumor on Rotella T is completely false. Lots of folks use it, love it, 100k miles + to prove it.

JUST MAKE ABSOLUTELY FOR SURE THAT YOU ARE USING THE API CERTIFIED CI4+ or CJ4 OIL!

Use the 5W-40 synthetic year round. It is great.