We use 5-40 Rotella in almost everything my family has. Trucks, heavy equipment, cars, boats, even small air cooled gas engines. We recently went 700 hours on an OCI for a Cat backhoe by mistake with no apparent problems. It only has an 8 quart capacity too.

The only thing we don't use it in is for cars calling for for 5-20.

Do you guys think there would be any problems using 5-40 Rotella in vehicles calling for for 5-20 other than a tad less mpg.(as long as they were out of warranty)?

Good question.....

Certain cars will tolerate this better than others - what vehicle are we talking about?

In modern cars and their engines, gaps and passages are much smaller between parts than they used to be.
If engine is old, and worn out,gaps should increase, and 40 weight oil should flow as 20 when it was new. In case of new, or good maintained engine, 40 weight oil might flow worse in tiny gaps and delay lubrication, which will lead to excessive wear and drop in mpg.
JMO

I wouldn't expect any problems. T6 is API SM-rated after all.

The vehiles I am talking about are a 2007 Mazda MX5 (Miata) and a 2011 Ford Fusion 2.5 Liter 4 cyl when the warranty is up.

Why do it? You would lose horsepower and mileage.

I seriously doubt there will be any problem with your choice of oil. I personally would be using Castrol GTX 5W30 instead because it is cheaper both at purchase & at the pump.

Neither vehicle will blow up b/c of 5w40. If you give it a try, please post back with the results.

I personally wouldn't use a 40 weight oil in a car calling for a 20 weight oil.

What do you think will be the result?
A Loss of 3HP at top end, and 1/2MPG in the city. Neither will be a devastating loss.

I doubt you'd have any problems, and might not even notice the difference, except for quieter operation.
Try it and see how it goes.
I'd bet you'd be fine.
How cold do you see over the winter in the Ozarks?

If you drive hwy and especially tow, go for it. If you drive city and especially short trips, too huge MPG penalty.

Does the owner's manual say "Use 5W-20, unless the BITOG folks say 5W-40 is OK"?

Just do it. You will in all honesty probably never know the difference.

Then again, 5w-20 dino juice can be had even cheaper than Rotella, so unless you want to pour Rotella T6 in everything just because it's on hand you can probably save a few bucks by buying PYB 5w-20 or Mobil Super 5000 5w-20 in 5qt jugs.