First let me explain the difference between fat and lean. Fat paint has more oil in it than lean paint. Lean paint is simply paint that has been mixed with paint thinner or turpentine. A good way to remember this is to pair these words in your mind: Fat = oil & Lean = Thinner.
The rule (especially when working with layers) is always to paint fat over lean. The most important reason for this is to keep your paint from cracking. Oily paint dries more slowly and is more flexible. If the fat layer is painted underneath the lean layers it expands and contracts while drying and the lean paint on top will eventually crack and flake. It may happen in a few months or it may take years but if you want your art to last --remember the rule--.
I tell my students to build the first layer (your foundation) with paint that is mixed very thin with turpentine or mineral spirits. You may use either water or a water mixable thinner if you are using water mixable oil paints. In some cases we may thin down the second layer also. Next, you can use paint straight from the tube. You'll probably mix your colors but try not to add any oil. Later layers have oil blended in. In the last stages of the painting there may be a great deal of oil and other mediums as we are often painting with transparent glazes.
Let me recap. When painting in layers start with a very fast drying under-painting. Thin your paint with turpentine or a fast dry medium. Next use undiluted paint just as it comes from the tube. Subsequent layers should have oil added You can keep adding more oil in successive layers but they should never have less oil than those underneath.