Viscosity, why does it matter?

Oil paint thinning and thickening
When we mix up oil paint, we sometimes make it thinner using turpentine or solvent so the paint flows more easily. Sometimes we thicken the paint using wax or impasto medium so that the texture of our brushstrokes is retained. Have you ever mixed up a batch of very thin paint for a wash, and then left it overnight in a sealed container? The next day the paint may still be fluid and usable, but often the pigment will have settled to the bottom or the liquids will have separated. Mixing up a more fluid paint for glazing reduced the paint's viscosity. This allowed the pigment particles to fall to the bottom.