When painting in oils, one must adhere to the principle of “fat over lean” to prevent cracking.  I think fat over lean might be better understood if considered as ‘flexible over less flexible’. One should therefore apply paint with a higher oil to pigment ratio (‘fat’) over paint with a lower oil to pigment ratio (‘lean’) to ensure a stable paint film, since paint with the higher oil content remains more flexible.  When painting in layers, the proportion of medium used in each layer should be increased. The higher proportion of medium makes subsequent layers more flexible and prevents the painting from cracking. One should therefore add more and more oil to the amount of solvent used. 

Oil paint dries at varying rates due to the differing siccative properties of pigments. However, everything else being equal, the higher the oil to pigment ratio, the longer the oil binder will take to oxidize, and the more flexible the paint film will be. Conversely, the lower the oil content, the faster the paint dries, and the more brittle it will be. Ignoring this practice, even in some alla prima painting, may result in a cracked and less durable paint film.

This applies to classical painting media such as turpentine, natural resins, and certain plant oils (linseed, walnut and poppy).  When applied properly, these will last for centuries.