It looks like you're new here. If you want to get involved, click one of these buttons!
There are currently no mediums on the market that slow the drying rate of oil paint adequately, so you will need to make it yourself.
recipe for slow-dry medium (for all colors except titanium white):
For burnt umber, you will need extra clove oil. Please watch this video for instructions on how to incorporate the extra clove oil into burnt umber: youtu.be/lpU9egKu-kM
recipe for slow-dry medium for titanium white:
* Venice turpentine is not at all the same as what is commonly known as "turpentine", a solvent commonly used by artists many years ago (and still used by some artists today). Venice turpentine, on the other hand, is simply tree sap — a thick resin which is thick like honey.
† Oil of cloves is sold as "clove leaf oil", "clove bud oil", or simply "clove oil" — any of these forms is fine. You may try looking for it in drug stores, health food stores, or from an online supplier.
Here is a video tutorial on mixing paint with medium: youtu.be/lpU9egKu-kM