Fat on lean rule
Oil Painting for Beginners - Rules
The 'Fat on Lean' Paint Application Rule
Because oil paint mixes with oxygen to cure then we must assume that if a pigment has higher oil content it will take longer to cure. To prevent cracking of paintings then we must remember that what is in the under layer must cure faster then what is on top. The under layers must be LEAN and the upper layers must be FAT.
We can control this using a lean mix (Less Oil than turps) in the under layers and a fat mix (more oil than turps) in the over layers. Prepared fat and lean mixes can be purchased from your art supplies shop and are a good easy option. Be aware that some pigments naturally contain more oils than others. Also be aware that lighter colours attract less heat then dark pigments and will oxidize faster.It's a juggling act that comes with experience and even then sometimes problems will result.
The general rule is do not use too much mix as all you really need to achieve is to loosen the paint slightly so it is easier to apply.
Pre-mixes for both fat and lean can be purchased at any good art supplies store and I prefer to use these for absolute ease of preparation.