Pure Tung oil is classed as a drying oil along with pure linseed oil. Pure Tung Oil is approved for food contact by the FDA as is pure lindseed oil.
Both tung and linseed are discussed in the comments. Linseed and flaxseed seem to be the same, but more refined is how I read it. But someone posted that they have used linseed with no problems.
I would not use tung or linseed especially since I have a good supply of rancid grapeseed oil. Rancid oil does not effect polymerization on the cast iron, apparently. Good use for something that would otherwise be tossed!
Off subject a bit but Be cautious of Tung oil and Lindseed oil when refinishing wood projects as it is extremely combustible when the soaked rags are left in a pile in the building. On the other hand keep it in mind if that drug house becomes a problem next door, especially if it is left under a pile of newspapers and kindling on their back porch...