I made a mistake once trying to season a 2 qt cast iron pan I did my polishing on the inside then started slowly heating it until it was just beginning to show some colorization to teh polished surface I added in regular vegetable oil not noting that it was of the non fat variety. The oil just got hotter and hotter until it finally flashed over after it was finished and I had cleaned up the pan with salt once it had cooled I noticed that it hadn't glazed like other pans and skillets had done. It was then that I realized it was the fats in which ever oil you use that has to caramelize into the open pores of the metal once they cool and close it is the trapped crystalline particles left from the fats that actually serve to prevent foods from sticking.
Just like was mentioned about the ferrous magnetite once it is polished off to become smooth then sealed with the fats from the oils
I've never used flax seed for anything and have only used linseed for treating wood so I cannot comment on its use
But I do know that pure old rendered hog lard was the preference back in the day. But you know your skillet is well seasoned when you can cut up a batch of potato wedges skin on and fry them to golden brown in just a table spoon of fresh churned butter without having to scrape them out of the bottom you have done your job.