Re: your 2˘ on sun-thickened. . .
I have seen a couple of videos of Richard Schmid, where he talks about his medium to achieve transparent shadows, and for general use when working Alla Prima, the recipe contains Stand oil:
1 Part Stand Oil
1 Part Dammar
5 Parts turpentine
I use stand oil extensively in a home made medium, and the drying time is 2-3 days when working fine layers, but maybe has something to do with the region where I live (Caribbean Sea).
This medium is prepared with 10% Amber varnish, 90% stand oil using high temperature for fusing the ingredients.. A slightly modified version is 1 part stand oil, 1 part linseed oil, 10% Amber varnish.
I can use it for general purpose, or for making paint transparent.