I keep my iron/steel molds (and aluminum), in a large ammo can in the garage with some desiccant powder. They have been out there many Michigan Winters without any visible rust whatsoever. The ammo cans have a rubber seal that keeps them air-tight.
I do not have to remove any oil or other preservative from them when I start to cast. I just remove the two (I always cast with two molds), I am going to use, and put them on the lip of the furnace to pre-heat as the lead melts.
When I am finished casting, I let the molds cool down until just warm and put them into the ammo can until I am going to do some more casting.