Welcome Silver1. (two days and 19 posts? you've been busy ...)
Let me break out of this silliness and bring in some reality.
The 7.3L is a stout engine; it's not going to self-destruct by using dino oil. There are rigs out there with 500k+ miles on them using good ol' dino oil.
Synthetics are useful for extending the OCIs. That is their most advantageous characteristic.
They can also offer extreme cold start advantages, but I can also tell you that is not a "needed" trait here (because I, too, live in IN as does the OP).
Depending upon the OP's OCI duration, and total annual mileage, he has a couple of choices.
If he does only one OCI per year - I'd suggest either 15w-40 or 10w-30 dino. The protection factor will be the same. UOAs prove it, even if some people don't want to admit it. I would tend towards the 10w-30, but some people cannot get past the "thin oil sucks" fear.
If he does more than one OCI per year, I suggest using a 10w-30 dino HDEO in winter and 15w-40 in summer. The thinner fluid will give better starting and less "romp" in the HEUI system.
And please allow me to cut you "shear fear" people off at the pass, before you even mention it. Oil shearing is common in HEUI; true. But it rarely, (probably never?), manifests into wear metal spikes; I have yet to see a UOA where the shearing of grade resulted in a jump in wear metals. Further, while a HEUI PSD will shear a 40 grade down to a 30 grade, it won't shear a 30 grade down much, if any, at all. It's as if the HEUI 7.3L and 6.0L systems simply prefer a thinner grade. There are some members here that run 10w-30 HDEO in their PSDs and have great success with good starts, low wear, and low contamination.
For those of you who fear thinner conventional fluid, because you don't think it's up to the towing task, why don't you read over my dino oil UOA, before you speak. Also review Ponch's UOAs, and others. I personally FLOGGED the living daylights out of my conventional Rotella 10w-30 oil last summer in the heat of UT and AZ, as well as the mountains of CO. I got excellent results, and could not have purposely been harder on my oil no matter how hard I tried. Read these:
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2323660&page=1
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1966220&page=1
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2425542&page=1
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2364435#Post2364435
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2179591&page=1
Those UOAs are just a small sample of what's out there. Dinos, thinner grades, Dmax and PSD and Cummins. They all show HDEOs capable. Heck, BigGreyMegacab is running 20k mile OCIs on dino oil! I tried my best to frag my lube, and short of setting it on fire with a torch, could not break it down.
The KEY to UNDERSTANDING is how they are used, and what the results show.
PUH-LEEZE do not spout off about "synthetics" are better, worse, etc until you read and understand the results from the environments in which they are used.
Silver1 - if you run long OCIs, then a synthetic might make sense. If not, then just use any quality dino oil you can find. The engine won't know the difference, but your wallet will. It would be my suggestionion that you run 10w-30 HDEO and see how it treats your engine; I think you'll be pleasantly supprised. Run a few OCIs, then get a UOA or two. Why not use your own personal data to decide, rather than a bunch of internet mythology? Further, you can peruse the HDEO UOA forum here. There are plenty of examples for you to review.
But hey, why let facts and data get in the way of HDEO synthetic mythology and rhetoric?
I now return you to your regularly scheduled synthetic lube bigotry and grade mongering.