Hello Milan, Conny's stuff is called 'Malmittel zur Olfarben' it's top stuff for matting flats. In additon, it drastically reduces the drying time as well. In my experience it gives oils a drying time similar to Humbrol enamels. And that's no exageration either.
Also Jay's suggestion is an excellent one as well and works very very well, as his great stuff testifies.