Federal law specifies that a manufacturer cannot void your warranty simply because you did not have the dealership or "authorized service center" do the maintenance, as long as you can document the maintenance has been done. Similarly it specifies that the manufacturer cannot void your warranty just because you did not use the manufacturer brand parts, as long as the parts you used meet factory specifications. So, you need to be sure that the oil you love actually meets the Ford specs. As long as you can document the work being done and the parts meeting spec, you cannot be denied warranty coverage. That does not mean you won't have to fight a bit to get it covered, but in the end it will be covered.
I too was a skeptic about how much better oil is nowadays compared to when I was a kid, but a number of samples being sent off to Blackstone, etc. and some research on how refining has changed has convinced me. I have no issues with running a full synthetic 10k miles and I have sample analysis to support that scientifically. You can change every 3k miles if you want, but you are literally pouring money down the drain.