Teaching a class right now with 17 people in 1 room for 5 days, I really needed to devise a solvent-free way to use oil paints. The method would have to still leave us with a day for wash in, and dry paint to work with every morning.
The solutions? Water-miscible umbers for the wash in... and M Grahams Walnut Oil Alkyd Medium for days 3 & 4. Water miscible paints DON'T act like regular oil paint + solvent, but they do a passable job, and remain open for several hours. The walnut oil is solvent-free, and makes paint very workable with just a few drops per inch of paint, then dries to a satiny finish for the next day.