Watch oils age. They oxidise, thicken, migrate away from the surfaces they were placed on, and gather the metal and dust particles circulating inside a case.
The result is a grinding paste in the places least able to tolerate it: pivot holes, jewel bearings and the escapement. Pivots wear oval, jewel holes enlarge, and the train loses the freedom on which the escapement depends.
This is why 'it still runs' is not reassurance. A movement running on degraded oil is consuming its own bearing surfaces, and the cost of the resulting repair is far higher than the cost of a service.
Correct practice is full disassembly, cleaning, inspection and fresh lubrication of specified points with appropriate oils and greases. Oiling a dirty movement through the plate adds solvent-thinned contamination and accelerates wear.
This is professional work. Disassembly of a mainspring barrel, removal of a balance or adjustment of an escapement by an untrained hand routinely causes damage that is expensive or impossible to reverse; take a valued movement to a qualified watchmaker.