I am currently developing various oil: water tempera paint formulations.
For example: a small amount of commercial watercolor (from a tube) was placed on a palette and an equal amount of water was added to the paint. % drops of 'Span -20' (a commercial non-ionic surfactant sold by Sigma-Aldrich was added and mixed to uniformity (a bout 2-3 minutes).
About 5-7 drops of 20-30% dammar gum ( m/vol in linseed oil) binder was added and , once again, stirred to a uniform creamy paint formulation.
Below is the drying paint applied to bristol paper:
it becomes dry to the touch in about one hour at 20 deg.C.
I clean the paintbrushes with 'Murphy Oil Soap" followed by rinsing with warm water. The paint does allow easy clean-up and it can be applied to undercoat oil pastels for creative blending.
I will upload more information as I proceed.