This page lists the available oils in The Witcher 3: Wild Hunt.
To increase the effectiveness of blades, a witcher can coat them with special oils he creates. These are tailored to damage specific types of foes. Oils that assist against humans or animals should be applied to steel swords, while any monster-specific oils are applied to your silver swords. There are as many oils as there are families of monsters, and so it falls to a witcher to be prepared for these eventualities.
Crafting oils in Wild Hunt is done through the alchemy menu while out of combat, but they can be applied to weapons at any time. Similar to potions, once an oil is crafted, it remains in your inventory. You can find formulae for improved versions of oils, requiring the previous level of the oil and new ingredients.
This again follows the standard, enhanced, and superior quality scheme used by potions. Oils do not last indefinitely. Each successful strike on an enemy rubs some of the oil off the blade until eventually there is none left and the effect is no longer applied to the foe. You can reapply the oil by going into your inventory once again, as long as you have not discarded it.