This article is loosely taken from F. J. Butter's Encyclopaedia of Locks & Builder's Hardware, Published by Josiah Parkes & Sons Ltd.,1958.
Many locks when manufactured have their heavier internal parts copiously applied with a fairly thick grease of good quality which is long lasting and does not thicken with age. However this will ultimately dry out and if any lock appears to require oiling, through difficulty to open or otherwise that may be done. But two things to guard against are a lubricant which thickens or becomes sticky after a time; and a mixture of lubricant and anything else that would clog the mechanism.
Lubricating grease for locks, other than Vaseline may not ordinarily be found in the home. So the use of oil when needed can be met by a general lubricant of high grade thin oil of the type used for sewing machines. This kind of oil is not likely to do harm between close fitting levers, and therefore may be used for warded and lever locks - albeit brass levers generally function for many years without ever requiring lubrication. A small quantity of oil, often just a film, is enough when needed. Although locks made wholly or partly of steel and iron, especially if such are used as working parts, need more frequent application than other locks.
However, pin tumbler mechanism (cylinder) locks should never be lubricated with oil. The right thing for them is a fine flake lubricating graphite which usually comes in a plastic 'puffer' with a nozzle or spout to insert in the keyhole. One good squeeze usually sends enough in the lock. This is also a good lubricator for other locks.
Weather conditions sometimes cause locks, which are fixed on doors in exposed positions, to work stiffly, even with brass bolts. To ease a stiffly working lock, it is often enough to put a thin film of oil on all sides of the bolts, provided they are clean - and in addition place a little oil on the drill pin of the lock and nose of the key.
When a spring bolt fails to strike easily on meeting the staple or striking plate, a smear of oil or Vaseline on the striking surface is usually an effective remedy.
For disc tumbler locks, thin light oil is a proper lubricant and a small amount squirted through the keyhole is enough.