There is so much information about medium recipes, that you need to simply try them out and see if any work for you. These are the recipes that work for me, I store them in little jars for up to 1 month (after which the Turpentine would have evaporated entirely). The longer they're stored for, the "fatter" they get.
NOTE: I now use Gamsol instead of Turpentine! Much less harmful and can be used indoor (windows open of course).
I have tested the below recipes with Winsor and Newton Artist Oil Colours:
For “alla prima” (painting in one layer) or “blocking in” (2 layers) colours will stay fresh for 3-5 days using this:
2 parts Gamsol; 4 parts Liquin; 2 parts Damar Varnish; 2 parts Linseed Oil
For “alla prima” (painting in one layer) or “blocking in” (2 layers) colours will stay fresh for 1 day using this:
Liquin (normal, smooth effect) or Liquin impasto (for thick layers of paints where the bristles show)
Other medium recipes and colours for painting in layers (fat over lean)
"Lean" medium:
5 parts Gamsol
1 part Damar Varnish
1 part Linseed Oil
"Medium" medium:
2 parts Gamsol
2 parts Damar Varnish
1 part Linseed Oil
"Fat" medium:
2 parts Gamsol
1 part Damar Varnish
2 parts Linseed Oil
"Very Fat" medium:
4 parts Liquin
2 parts Damar Varnish
3 parts Linseed Oil
"Alla Prima" medium:
2 parts Gamsol
4 parts Liquin
2 parts Damar Varnish
2 parts Linseed Oil
"Quick drying mediums" for "alla prima"
Liquin (fine details)
Liquin Impasto
Mark Carder's slow drying medium recipe (I haven't tried it yet but I trust him entirely):
One thing I noticed is that while I use Geneva Oil Paints (manufactured by Mark), which are perfectly already mixed when freshly bought, is that after you have opened them for a month or two they do need some medium. I use linseed oil with them and it works very well. Just be aware of it if you invest in them.