Richard McKinley is the king of oil washes and he's so skilled with the underpainting that many times he leaves quite a bit of the oil wash showing.
In this example you can see that I have covered almost all of the oil wash-not as skilled with my underpainting as Richard McKinley. However there is still an advantage to the oil wash method.
When an underpainting is created with a very thin application of oil paint, thinned to a stain with turpentine, the initial application of pastel sticks like glue to the sanded paper because there is no underpainting of pastel - no layer of pastel to fill up the tooth of the paper.
The layer of pastel is applied with a very light hand watching carefully to allow the creative play of shapes and colors to emerge from the oil underpainting.