We have been taught from an early age that oil and water do not mix . . . and then along comes water mixable oil paint. How did that happen and more importantly, why?
Why: Paint manufacturers wanted to create a product that gives the luminous result of oil paint but without the fumes, handling and environmental issues associated with turpentine and solvents used for thinning and cleaning.
How: An emulsifier was added to the oil paint to allow it to be mixed thinned with water.
It's important to note that although this paint has been chemically modified to accept water, there is a limit to how much water can be added so it should not be named water miscible or water soluble, terms that imply the paint can be thoroughly mixed with water. Instead of using water for mixing and thinning, why not try to use special medium that has been created for use with water mixable oil?
Other notes:
I find that the water mixable oil does not have quite the same consistency as traditional oil paint.
Water mixable oil dries a little darker if water is used for thinning or mixing but not quite as much is modified medium or linseed oil is used.
Because both water mixable oil and traditional oil are oil paints they can be used on the same painting - but - you can't use water.
Hope this helps! Enjoy painting.