Oh and on the soapy water debate - you can use soapy water no problem, and this is no bad thing every now and again just to get off any stubborn dirt. Unless you are scrubbing the hell out of it you aren't going to wreck the seasoning.
I find flax oil to expenpensive to use for frying or treating cast iron. I use it in cottage cheese as cancer perventative and on salads with vinegar. I store it in the fridge and never heat it. It is high in omega 3 essential fatty acids but heating reduces that. The only foods that are higher in efa's are chia seed and the edible algaes.