Ford with a 9 year warranty? Assume the oil outside of the weight is the same, not mixing conventional with Synthetic.
Also sounds like you need a Seafoam treatment, still burning carbon, builds up on piston rings, lets oil slip by. Also carbons up spark plugs that is conductive on the insulator, shorting out your spark decreasing your performance and fuel economy. Can mix one ounce of Seafoam with each quart of oil to clean your hydraulic lifters before your next oil change.
Devils advocate , oil change places throw in what ever big bin oil they buy unless they upsell you to synthetic when your rig has conventional or blend. Also they do not know if you even have the correct oil in your vehicle. Always pull over right after an oil change and look for leaks, on the ground.
I also have my , fords engines of this era just dont like anything else but 5w20 synthetic. Adding a quart here and there of other stuff, not really a problem, just maintain that engine with real good syn 5w20 and just wait for the rest.
It was not specified what engine is in that flex but I am suspecting it is your basic 3.5l duratech, they dont like anything other than 5w20.
But the disclaimer here is I live in a region where temps vary GREATLY, so I do roll some cop cars with same engine, with 5w30 when the variable cam system gets some wear, but usually the police packs come with the 3.7 awd, same engine basically when you look at it, all those get 5w20 mineral, cuz they idle so much, so we cant go by normal interval on these fords.