Do not use WD-40!
Its protection is very short-lived and its best alternative use to its intended water-dispersant as its name shows, is for flushing old lubricant out of bearings!
It is just a very thin oil diluted with white spirit.
You are best using ordinary lubricating oil or grease, or a protective spray or brush applied oil, like 'Duck Oil', made for such work. Some types slowly dry to a protective film that needs be wiped off with a cloth moistened with white spirit. These protectives are used on the machines imported by firms like Axminster and Warco.
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For longer use on static surfaces, petroleum jelly ('Vaseline' and lanolin are tried and tested materials. An old way to protect wire-ropes is to soak them in a solution of lanolin in meths or white spirit; but note this is not a lubricant. It is a water-proofing that also has the advantage of being safer on the skin than mineral-oils (it was once used for protecting the steel-wire ladders used in caving).
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Covers? I am not sure if this is always wise as an impermeable barrier can trap water-vapour under it. I leave my machines uncovered but lightly oily, and they seem fine; in my unheated but moderately well insulated workshop. Unprotected steel seems to rust only a bit less in the house!
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Samuel Haywood mentions linseed-oil on garden-tools. Any vegetable oil would be entirely appropriate there, and you could use linseed-oil for other tools, but is likely to dry to a sticky residue.
Year back I encountered castor-oil filling old-fashioned sonar transducers, where it conveyed the signals very efficiently between the active element, through a rubber diaphragm, and the surrounding water. The fluid in our cochleas, and the spermacetti and similar fatty oils in marine mammals' hearing, are the biological equivalents. Castor-oil though, has an affinity for any and every nearby surface, where it sets to a tenacious glue horrible to handle and to clean off!