Different oils used as mediums in oil painting are referred to as drying oils.  This is due to the different drying times and properties.  They are mixed with oil paint either to modify the way the paint handles directly from the tube or to alter the paint’s character from what is the effect coming straight from a paint tube.  Ideal mediums do not have any color, are permanent and flexible, and does not have any effect on the pigment’s color.  Knowledge of the particular properties of an a drying oil is part of the important technical knowledge that an oil painter should possess. 

Linseed oil comes from the seed of the flax plant.  It adds a glossy effect and transparency and is accessible in many forms.  It dries very thoroughly.  Refined linseed is a well-known, multi-purpose pale to yellowish oil which has a drying time of three to five days.  Cold-pressed linseed oil has a faster drying time than refined linseed oil.

Stand oil is a thicker processed variation of linseed oil, with a slower drying time. It is best used for glazing and creates an enamel-like finish without any brushmarks.

Sun-thickened linseed oil is thick, syrupy, a bit bleached oil produced by its exposure to the sun.  Its ability to thicken depends on the climate where you reside.  It is yellowish when it dries.

Sun-bleached linseed oil is produced through the oil’s exposure to the sun with the container’s lid on so no evaporation happens.  As a result, there is little tendency for the oil to yellow. 

Poppyseed oil is a very pallid oil, clearer and less likely to yellow compared to linseed oil. Consistency is its effect on oil paints.  Compared to linseed oil, it has a longer drying time of five to seven days.  It contains 50% natural oil. 

Safflower oil is similar in characteristics with poppyseed oil but has a quicker drying time.  It comes from safflower seeds. Likewise, sunflower oil also possesses the same characteristics of poppyseed oil. It comes  from sunflower seeds.

Walnut oil is a pallid yellowish-brown oil  known for its distinctive smell. It gives oil paints a fluid effect.  It is ideal for pale colors because it yellows less than linseed oil.  Its drying time is four to five days.  It is quite expensive and must be properly stored otherwise it becomes rancid. It contains 65% natural oil. 

Boiled oils are oils that have been heated and blended with a drying equipment to produce a smooth finish and faster drying time.  As the oil gets old, they have the tendency to yellow and darken so it is suited to lower layer painting and darker shades.  When uncertain about its effects, spend some time testing rather than destroy the whole painting.