There's wisdom in the adage "apply thick paint, thinly." Working with paste paint in a thin layer is often the most effective way to use oil paint. A high-quality tube of oil paint is designed with the ideal pigment-to-binder ratio, known as the Pigment Volume Concentration (PVC). Adding medium, oil, or solvent can alter this ratio and dilute the pigment, potentially affecting the final outcome of your painting.
Oil binders in oil paint may yellow over time, with stronger binders like linseed oil experiencing the most yellowing. However, this yellowing is typically masked by the pigment load in well-made oil paints, so it is usually not a significant concern. Yellowing becomes more noticeable when you dilute the pigment concentration by adding additional substances, especially in lighter colors like whites and blues. For these colors, it's recommended to use a less yellowing, albeit weaker, oil like poppy or safflower.
Excessive use of oil can cause several problems: extended drying times of over five years, wrinkling in certain areas, and a surface that attracts dust. To avoid these issues, use as little medium as possible. Alkyd mediums add flexibility and are a solid choice, while solvents can reduce the strength of the paint film. For a transparent glazing effect, try diluting your paint slightly and applying it in a very thin layer.
Avoid mediums containing soft resins (dammar, mastic, larch Venice turpentine, balsams, etc.) as they can make paint weaker, more brittle, and prone to cracking and flaking. Additionally, these mediums may not harden enough to resist solvents when varnish is removed. Instead, consider using a small amount of an alkyd medium.
Ultimately, the key to a great painting lies in the artist's vision and mastery of pictorial elements, such as tonal values, contrast, composition, drawing, color coherence, depth of space, brushwork, repetition, rhythm, line, and shape. Keep your focus on these aspects to create a truly remarkable piece of art.