Since there seems to be some confusion over whether Ford has "the right" to deny a warranty claim, hopefully this clears some things up. Let's compare this to engine oil. Let's say you have a car that, according to the automaker, requires 5w30 oil. Let's say it's a basic economy car engine that doesn't have any special requirements or specifications the oil has to meet, aside from the latest API spec. (think: oil requirements for a 1998 Ford Escort). Now, the automaker isn't going to say, "You MUST use Brand X oil, or the warranty will be void," and they probably don't even care which brand oil you use. Just make sure it's 5w30, and you're good to go. Now let's say that, for whatever reason, you decide to use straight 60 weight oil instead, and on a cold winter morning, the super thick oil doesn't flow adequately and the engine seizes as a result. If the automaker finds out the wrong oil viscosity is in there, AND they determine that the engine failed BECAUSE you used the wrong oil, THEN they'll deny warranty coverage.
Now let's say that even though you're using the wrong oil, the engine still runs fine. But one day the water pump starts leaking. You could tell the dealer you use straight 60 weight oil, and that will NOT void the warranty. Because the water pump leaking has nothing to do with oil viscosity; it would've leaked even if you used the correct oil. Again, even if you use parts or fluids not approved, you can only be denied warranty coverage if those parts/fluids CAUSED the failure.
So, getting back to the transmission: Think of Mercon LV as 5w30 oil. The engine in the above analogy can use ANY brand of oil, as long as it's 5w30. This transmission can use ANY brand of ATF, as long as it's a Mercon LV fluid. Ford does NOT care which BRAND of fluid you use. You do NOT have to use Motorcraft brand. But you DO still have to use a Mercon LV fluid* (*see note below). If you put ATF +4, or Toyota WS, or a CVT fluid, or some other wrong fluid, AND Ford determines that the transmission failed BECAUSE you used the wrong fluid, THEN they'll deny warranty coverage. If it failed for some other reason, (in other words, it would've failed no matter what kind of fluid used, correct fluid or not), then they cannot deny warranty coverage.
**Note: When I say you must still use a Mercon LV fluid, I mean (more specifically), you have to use an officially licensed Mercon LV fluid. That's because Mercon LV is a spec that was made up by Ford. Ford is the one who decides what is and isn't Mercon LV, since they're the ones who invented it. An oil company cannot mix up their own batch of fluid using whatever ingredients they want and claim it's a Mercon LV fluid, *cough* Valvoline *cough* Maxlife ah-hem *clears throat* and expect Ford to just accept it, especially when Ford is the one paying for warranty repairs. But if it's officially licensed, then that means Ford actually tested it themselves, saw that it meets their specs, and approved it. Again, this is not the same thing as saying, "You must use Brand X." You can use any brand fluid you want, as long as it's an officially licensed Mercon LV fluid.
Maxlife, as we all know, is not an officially licensed fluid. Now that doesn't mean it's a bad or poor quality fluid. It simply means Ford hasn't tested it or officially approved of its use in their transmissions. That's it! If Valvoline wanted to, they could ask Ford to test it and license it at any time. And for all we know, if that happened, maybe it would pass all of Ford's tests with flying colors and exceed their specs more than any fluid before. Or maybe it would fail miserably. Nobody will know until it actually happens. And that's why IF the fluid was the cause of failure, or could've contributed to the failure, Ford wants to know if it was the proper (approved) fluid. It doesn't matter how many people have used it without problems. It doesn't matter if Valvoline "recommends" it for Mercon LV applications. I could pee in a bottle and I could "recommend" that you use it in your transmission and sell it to you for $6/qt. The only thing that matters is that Ford officially approves of its use in their transmissions.