Mineral oil twice a year is good.

by Ed
(Brenham, Texas)

Have used plain mineral oil on my carbon steel broadsword blades for 18 years with never a spot of rust. A bi-annual application is enough as long as the the swords are rarely and carefully handled. Downside is that the oil traps dust and lint in the air. This can be unsightly, but doesn't seem to compromise the protection.