I "fixed" this... And in all my years of wrenching cars and owning the occasional beater, this is a first.
So as I'd been saying, It smelled like oil, but raw oil - not burning oil (which I am intimately familiar with from owning shitty cars as a kid) and not gear oil (which everyone knows smells like absolute ass). I thought it was associated with my heater, because I'd notice it immediately get VERY strong as soon as I turned the heat on. I'd also notice it when I'd roll the window down.
As it turns out... It's because of some kind of oil that smells JUST like motor oil that Goodyear puts on their all weather rubber floor mats. Or more accurately, that was on my mats, and all of the mats just like it at the Autozone
that I bought them at. (The mats are new, since I've only had the truck a week.)I pulled the mats out to clean them (because I assumed the heater was blowing oil on them) and when I was driving around without them I noticed the smell was gone. So I took a trip up to Autozone to (comically) smell the other mats there, EVERY one of them smelled JUST like my mats.
So my heater blowing hot air at my feet was heating up the oil on the mats, making it vent all over the cab. When I'd roll the window down, the air circulation blowing into the footwells would waft it up at my face.
So after a week of wondering where the holy hell this oil smell in my truck was coming from, and why the inside of my truck smelled like oil but not my engine bay... I got new floor mats and problem solved.
Weirdest fix I can remember.