There is no one "best" oil for cast iron. However, some oils that are commonly used for cooking, such as olive oil or canola oil, are not ideal for use on cast iron because they contain high levels of polyunsaturated fatty acids (PUFAs), which can damage the metal. Some oils that are specifically designed for cast iron use, such as corn oil or grapeseed oil, are much better choices because they have a lower level of PUFAs.