I Use a traditional oil-varnish medium when plein air painting, based on the formula given in Ralph Mayer's Artist handbook of materials. The varnish concentration used in his formula is 5 pounds of damar cystals per gallon of turpentine (called 5 pound cut). I make my own varnish to be sure that the varnish concentration is correct.
However, I developed a very quick and simple way to make the medium from stand oil, damar crystals, and turpentine or mineral spirits as given below. This method eliminates the slow and messy step of making the 5 pound cut varnish. The method seems to work with mineral spirit instead of turpentine because the damar crystals are melted into the oil before any solvent is introduced.