Gamblin Solvent Free Fluid
I cannot recommend this incredible product enough! It has the exact consistency of traditional linseed oil, yet it dries much faster - usually within a day and without VOC laden driers. Gamblin has managed to make a non-toxic alkyd medium. This is progress! It is made of alkyd safflower oil, so it is very good for not yellowing over time. It leaves a strong paint layer, and as is linseed oil, it is a cinch to work with - easily changing the viscosity of paint, limbering brushes and with imprimatura applications, limbering surfaces for better slide, and can be used, as all mediums can, as an alternative to odourless mineral spirits to clean out your brush within your painting session, when going from one colour to another.
This is modern technology at its best! Once again, Gamblin has hit a home run.