DontPanic42
Adventurer
Never use butter to coat cast iron cookware after washing. It being an animal product, it can rot over time and could potentially make you sick. It too normally has salt in it which can make the cast iron rust, which negates the whole idea.
As far as spray can oils, the solvents in them, i would personally not be putting in my food.
The only thing that touches my cast iron cookware, is vegetable oil, canola oil, that type thing. Anything else could lead to trouble.
Just my $.02 worth
Even vegetable oils will become rancid in time. We have been using mineral oil on all the cast iron we use on our chuck wagon. In nearly 20 years, we have never had a pot go rancid. A liitle heat will vapourize the oil so it doesn't leave a residue. It can't be used to season the iron, however. It does keep it rust free and odor free between uses.
A little bit goes a long way
Happy cooking