From the suggestions, it appears there's some extra unassessed variables involved.
Airborne dust? Temperature extremes? Something.
And I think Declaration Day is talking about the door latches now, instead of the actual delicate key-lock mechanism, with the lithium grease.
I guess theoretically, the best treatment would be to blast out all previous and gummed up lubricant and goop with a zero residue solvent (alcohol, ether from starting fluid?), then lube with a dry, stable lube like graphite or maybe moly di, which won't let dust stick to the mechanism.