YSK: If you make popcorn in a pot on the stove, the oil to use is ghee.
Why YSK: Popcorn fans often want a buttery flavor, but plain butter is a bad choice for popping popcorn in a pot, because the proteins and sugars smoke and burn around the same temperature where it's hot enough to pop the kernels.
Ghee, or Indian-style clarified butter, is butter that's been simmered and the milk solids (proteins and sugars) skimmed off. This leaves a clear yellow oil that doesn't smoke when it's heated and doesn't go rancid quickly, but has a distinct toasty butter flavor.
Vegetable oil is either flavorless or faintly bitter, and some high-temperature vegetable oils tend to start polymerizing (i.e. becoming plastic) when heated in small amounts. This is also not good for popcorn.
Good-quality popcorn popped in ghee reliably produces lots of "butterfly" popcorn with few unpopped "duds" and no scorched kernels or batches ruined by smoke.
As I mentioned in a comment above, I have one of those theater poppers. But in those, the flavacol goes into the hopper right along with the kernels and your oil of choice. It's not an oil substitute. It's good stuff, but hard to find in less than 5-pound boxes.