One small drop on each side of the pivot, then work the blade twenty or thirty times and wipe away everything that comes back out. Use a light machine oil or a purpose-made knife lubricant. The commonest mistake is using far too much, which turns the pivot into a lint trap.
Open the knife. Put one drop where the blade meets the handle on each side, right at the pivot. Cycle the blade repeatedly so the oil reaches the washers or bearings. Wipe off everything that appears on the outside.
If oil is visibly pooling anywhere, you used too much.
A light machine oil, sewing machine oil, or a knife-specific lubricant.
Avoid heavy oils and thick greases. They attract dust and lint, and a pivot full of oily grit is stiffer than a dry one.
When the action feels dry or gritty, not on a schedule. A pocket knife in an office needs it rarely; one used outdoors in dust needs it often.
If oil does not fix gritty action, the pivot needs flushing rather than lubricating. See fixing a knife that will not open smoothly.