At a very first time I didn't thin my oil colour as I read in some tutorial books and articles on various magazines. Now, and after having seen and spoken a lot with some friends of mine that with oil paintings make miracles ( i.e. Gianfranco Speranza) my technique is a little bit changed.
I use as thinning a Quick drying petroleum medium from Lefranc & Bourgeois only and only this medium. It helps the colour to dry perfectly matt, sometimes also without the classic oven drying. It is clear that if you use too much of it, the covering properties of the colour could be less, but I haven't never a problem with this method of thinning.
Try to not thinning a red or a blue, then tell me if the final work you made with them is perfectly matt (expecially if you paint napoleonics, as me)...
Good work.
Marco