Grapeseed oil.
Cheap at big box stores. Incredibly high smoke point. Dirt cheap.
Use that for first couple layers and after that honestly whatever oil or fat you want to use or have. I re-up my pans with everything from Crisco to just cheap “vegetable” oil (rapeseed or soy usually) and even duck fat from after making other dishes.
Don’t expect any to necessarily be more delicious but sometimes you get different flavors from what sorta burns in. You supposedly might get some iron passing through but it’s actually kinda a necessary mineral like volcanic ash.
Yeah, to clarify, I was asking about the iron because I understood it to be a generally good thing but then questioned whether it really was an advantage of cast iron pans at all.
Though for the grapeseed oil having a ridiculously high smoke point, wouldn’t adding a final layer of something else mean that that will have the relevant smoke point?