It contains only the little oil present in the amount of paint needed for the color to be seen. After the application of this first very thin layer, it looks almost watercolor on paper. You can string together several diluted layers of different colors if you want to use the fun technique of “glaze”.
The next layer is “alla prima”, that is to say using directly the paint output of the tube as you did for the color study. You then have an average fat level, much like a person who is neither fat nor thin. Afterwards, the more oil or liquin you add to the paint, the more greasy it will be. If you paint “lean on fat”, you will have problems because the fat layers take the longest to dry so they should be applied over the less oily layers. Otherwise, the surface layer will dry faster than those below and the paint may remain trapped and not dry.
In the worst case, a painting that has been painted “lean on fat” can slip from the canvas if it is hot because the paint does not adhere. It happened at least once to a former student of a teacher who told this anecdote.
Never use oil pastels under the oil paint as these pastels contain mineral oil that never dries. If you want, you can add bold pastel markings on the last coat of an oil painting once it’s dry to the touch.