Join Date: May 2016
Location: Stockholm, Sweden
It depends on what you mean by "medium". If you use too much solvent, then it might cause flaking, because the binder (linseed oil) becomes too thin. On the other hand, if you use too much linseed oil, then it gives rise to wrinkles in the paint, which is what has happened to Watteau's paintings. But you have to be really careless to create such calamitous effects.
Adding medium cannot cause cracks if it's a good medium, containing both binder and solvent. The only thing that happens is that the paint layer becomes more transparent. It allows light to penetrate the layer and bounce around in there, creating trompe-l'œil effects. Painting directly out of the tube means that the pigment particles are so densely packed that they won't allow light to penetrate. It is also wasteful. You get twice(?) as much paint for the money if you use medium.
The medium in oil paint tubes is sufficient to create a strong paint layer, with one exception. It has always been known that ivory black will start flaking if you don't add medium to it. I made that mistake myself. But all pigments are improved with some medium added to them. It depends also on what brand you use and which pigment. Some already have much binder in them. It is also the question of what artistic impression you want to make. If you want a strong white highlight, then you shouldn't mix it with linseed, as it turns more yellow with time.
However, most artists don't bother much about these things, I imagine. Oil painting is a remarkably durable technique, and all brands are good today, even the cheapest Chinese brands.