Winsor & Newton drying oils, used in conjunction with oil colour, will modify the colours existing consistency, gloss, and drying times. Using oils maintains the flexibility of paint films, particularly when working fat over lean, and prevents over thinning, which can happen when using solvents.
Refined Linseed oil is a low viscosity alkali refined oil of pale colour, that dries slowly. Reduces the oil colour consistency, and increases gloss and transparency. Add this oil to your colours to slow drying time.