Put some Venice turpentine into your everyday turpentine (10:1 turpentine to Venice turpentine). You could substitute larch turpentine or Canada balsam for Venice turpentine, both of which are easy to obtain.
The admixture of Venice turpentine firms the turpentine without overpowering it. I sometimes use the mix to thin paints, and as an additive in my medium.
Why? I want my painting medium to be as flowing as possible. Turpentine-thinned oils are flowing, of course, but can lack firmness and precision. The Venice turpentine acts as a corrective and slows drying times for mediums that are otherwise too fast-drying. I use driers as well, but that’s a discussion for another time.