When tubes of artist’s oil paints are displayed for sale, they are usually hanging down in a rack, with their caps hooked on top.  The labels on the tubes are meant to be read with the tube caps pointing up rather than down.  This is great for a retail display, but perhaps not the best for the contents inside the tubes.  The reason has to do with the way paints are made.  Many different pigments are used in making paint, and each pigment has its own unique characteristics.  The particles of some pigments are very fine, but many are much larger.  When various pigments are ground in oil, they behave quite differently.  Some require large amounts of oil, and others require very little.  Often there is a surplus of oil that rises to the surface of a tube when the heavier pigment settles to the bottom.  This does not mean the paint is of poor quality – it may actually be an indicator that the maker of the paint has used only pure pigment, without added fillers. 

Paint manufacturers occasionally  add stabilizers such as aluminum stearate to prevent the oil from separating, but this is not necessarily an indicator of poor quality. It turns out the best way to store tubes of paint is not with the caps up, nor is it with the tubes on their side.  The best way to store artist’s oil paint is standing up with the caps on the table.  This way, the oil can rise to the top, and when you open the tube you get the pigment you need, without the extra liquid.  Knowing this, some manufacturers use extra large caps on their tubes!