How I clean brushes when using H2O soluble oil paints.
by Ray Roberts
In a brush cleaner (a jar with a metal coil in it) I mix a very small amount of laundry soap with warm water. This dissolves the paint out of the brush. Using a second brush cleaner with just water in it rinses the soapy water out of the brush. A very simple, quick and effective 2 step process. In my experiences, water alone does not effectively clean H2O soluble oil paint out of a brush.
I am not sure how soap would effect paint. For example if the soap concentration built up in the second (rinse) brush cleaner, it would transfer to the paint when using that brush again. It would probably be a good idea to refresh the water often in the second brush cleaner.