Oils can be defined by their drying capacity as: drying, semi-drying and non-drying oils. These definitions can help you choose which kind of oil is best suited for your hair care routine.
What does this mean?
Here is some information on oils by Transport Information Service on the definitions.
What does this have to do with hair care?
Quite a lot actually. While some oils can be difficult to remove from the hair because of the amounts used on the hair at one time or the method used (shampoo versus conditioner only), semi-drying and drying oils present more of a problem (semi-drying oils are not too much of a problem but they can still cause build-up), due to their chemistry and reaction to oxygen.
In the case of drying oils, the information on the cleaning of the oil tanks matches the problem that that you can have using a drying oil frequently on your hair, the difficulty (need to repeatedly clarify the hair) to remove it. Drying oils become resinous when they are exposed to oxygen.
Drying oils polymerize. That applies to any drying oil.
Does this mean that drying oils should never be used on hair? No. You do need to know what you are dealing with, and the consequences of overusing such oils, build-up that can be problematic, leading to dry, brittle hair if not removed. Drying oils are best used sparingly, and less often than other oils on hair, to avoid problems.
Examples of drying oils: borage oil, evening primrose oil, grapeseed oil, hemp seed oil, linseed (flax oil), pine tar oil, poppy seed oil, red raspberry seed oil, rosehip oil, safflower oil, sea buckthorn seed oil, soybean oil, sunflower oil, walnut oil, wheat germ oil
Examples of semi-drying oils: corn oil, cottonseed oil, sesame oil
Examples of non-drying oils: argan oil, babassu oil, coconut oil, mineral oil, olive oil
The iodine value of an oil determines how the oil is defined and some sources differ as to the cut off point between the numbers for semi-drying and drying oils. You can look up an oil and its iodine value, or look up an oil by one of the three definitions.
If an oil has an iodine value of 130-190 or higher, it is a drying oil.
Here is an extensive list. Just click on iodine value.