Please advise how to lubricate stiff latch bolt of mortise lockset on metal exterior security door. (NOT the pins in the keyway!)
Yes, this is just a lump of metal, big enough that it doesn't matter a lot what you lube it with. (I'd lean more to a grease than an oil -- lithium grease used with a light touch, perhaps.) But it also shouldn't need lubrication on a regular basis, so I suspect you've got something else going on.