Try putting the Winsor and Newton (or any other make) oil paint on absorbent material for a while and allow the linseed (carrier) oil to 'leak out' before adding the thinner to make the wash. Linseed oil increases the time to dry out so artists have time to rework it on a canvas (and affects the sheen/matt too) however this is probably not helpful for model painting. You need the wash to run into crevices and details, making use of surface tension effects. Then you can take a clean brush with some thinner added, take out the excess with absorbent material, so it is just damp with thinner then 'pull' the excess wash back off into the clean brush, where you don't want the wash on the model. Just a suggestion.