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As long as it is 100% flax seed oil, you can use it for seasoning. There are different approaches in storing flax seed oil. One is the supermarket approach, which basically is, "We don't care if the oil starts tasting awful". Try one of those 100% flax seed oils that are not refrigerated in the store. They all taste very bitter (still not rancid). If stored properly, flax seed oil should not taste bitter at all. That can only be achieved by storing it at low temperatures right from the beginning.Also, flax seed oil should not be stored longer than 4-6 weeks. For seasoning a pan, the bitter taste is no issue.