Use a mixture of Gamsol and Galkyd, (or Galkyd Lite), 50:50 (1:1). Adding Gamsol to CWM isn’t quite the same, so for wash consistency ("drippy paint") use 1 part Gamsol to 1 part Galkyd  or Galkyd lite; this is quite thin and fine to add to your oil/cwm mix to get washes. You'll get nice fluid marks. This won’t dry to a glossy finish though, because you’ve thinned the Galkyd lite (or Galkyd) but enough binder to maintain the adhesion.  You’re also lessening the ability of the Galkyd to yellow your paints by adding Gamsol to the Galkyd; you’re thinning the alkyd resin in the Galkyd or Galkyd lite so it should be okay (ie, low-yellowing) since you’re adding to oil paints (which have pigment) anyway.