Good thread here with pictures: Can u use Single Platinum Spark Plug in 4.0 sohc?
Not that I ever thought about it but because of the waste spark system Factory used two different spark plugs, 3 with stronger tips and 3 with stronger electrodes, they are buying thousands so this saves money.
And IMO this is why some "other" brands might not last as long in the waste spark system, and why there can be confusion with part numbers.
Basically you want to use the double platinum SP-440, strong tip and strong electrode, so it doesn't matter which cylinder it goes in.
If another brand was also double platinum it may work as well
I would say you would have to get better MPG if all cylinders were firing normally, a misfire or even partial fire doesn't burn up all the oxygen in the cylinder, this oxygen is dumped into the exhaust system....................which is where the O2 sensors live, lol.
O2 sensors read oxygen levels, high oxygen = lean, so computer adds more fuel
So not only do you get slightly less power from a misfire, you also use slightly more gas because of it, double whammy