Here's my opinion.
I am certain that this is the underlying problem with this PTU.
The variability of failure in this unit, which in design is simple (albeit fatally flawed in terms of lubrication volume) points directly to this tire circumference theory.
There is no other practical explanation for this issue. If it were heat from the exhaust, then the failure would be in accordance from those that are driving long distances. In addition, it would occur in the first 10000 miles of ownership - especially for those who are driving long distances.
This is simply not the case - because the tires have not worn down during this interval to pose a problem...
When this problem reared it's head in my case, was near the end of my tire's lifespan. I did not rotate the tires as I needed to in this fragile scenario. That's when oil began to boil out, stick to the undercarriage to the car, then drip down to the garage floor.
Ford says that this is a non-maintenance / lifetime item. They are correct - as long as you rotate your tires every 5-7k miles.
In my particular case, I caught the problem in time and saved the case. There appears to be no bearing damage nor noise, and I flushed the sludge with a solvent of petroleum fuel oil. I then added a superior crankcase oil than what is recommended by Ford - specifically one with the highest boiling point that I could find. I now know what to look for, and expect that with faithful tire rotations this will no longer be a problem.
I'll let you all know how this works out, and if any more oil is boiling out after the fresh set of tires and the new AMSoil oil...