Liquin is invaluable for providing this, transparent glazes, its "sticky" nature makes it quite different from traditional oil thinning mediums and its also helpful for refreshing dry oil colours, especially darks while a painting is still a work in progress.
Makes painting in oils MUCH easier. Be prepared for it to dry a little sooner than you might want, in some applications. Good for multiple-layer paintings.....tends to dry too quickly for very smooth glazing. Give it a try!