This is something that I learned not so long ago quite by accident. I didn't learn it in school, or a book, but by chance.
I was painting and I had bought some new oderless mineral spirits. I put the brushes and the old into seperate containers. One had turpentine and mineral spirits mixed together. I put a brush in this to move around some extra thick oil paint and it worked perfectly.
The best way to thin oil paints for a smoother coverage is not to use a medium, but to use terpentine. Some of the paintings I posted recently I used this method with including Blue Madonna and The Last Frontier.