How to treat a cast iron pan that only gets occasional use
My (carbon steel) wok is used with that sort of frequency and doesn't go rancid.    What I do is (you may want to modify a little for the fact you're on cast iron):


Rinse and dry it.
Get it fairly warm.
Pour in a little oil (sunflower as that's what I mainly use).
Swirl/wipe the oil over the surface.
Heat until I think it starts to smoke; turn off the gas.
After a few seconds wipe of the excess with a paper towel.
After a few minutes wipe it off again as some oil will have magically appeared
Heat until it just starts to smoke again (fairly slowly to allow the heat to even out).
Allow to cool before putting away.


Now this isn't a very hard seasoning, but it does a good job of stopping the pan from rusting and stopping food from sticking next time it's used. The final amount of oil is very small, which helps.