I use Venice Turpentine in my medium. I have bought two brands and they were dramatically different - causing me no end of headaches and some fair amount of research - here is what I found.
Venice Turpentine is the resinous sap from the European Larch tree. It is called Venice Turpentine because it was traditionally shipped from the port of Venice. Sources I found seem unclear as to when it first saw use in painting, but sources suggested between 1400 and 1500.
Venice turpentine is added to painting mediums, and it is a component in certain natural glues and varnishes - specifically for horses hooves - still in wide use today.
Added to paint, or in a painting medium, it imparts a definate enamel like gloss to the paint and strengthens the paint film. Care must be used not to add too much of the stuff or cracks and yellowing may occur.
I bought two brands of Venice Turpentine - Talens and Sennelier.
The Talens stuff is called Venetian Turpentine. It is straw colored and thick. The ingredients list it as Oil of Turpentine. It dries somewhat slowly, but not too bad. This is because it has very little actual Venice Turpentine in it, but is actually thinned down with Oil of Turpentine or in plain english speak - Gum Spirits of Turpentine.
The Sennelier stuff is called Venice Turpentine. It is a darker color - sort of amber and also very thick. The ingredients list it as Pure and Natural Balsam. It dries very, very slowly, such that you really need to add a dryer like Cobalt Drier to it - 10-12 drops per 6 ounces of medium. It appears to be the real stuff - not diluted or thinned at all.
My medium with the Talens stuff dried in about 3 days. When I started using the Sennelier VT, the drying time went out past 5 days - hence the drier.
Given a choice, I am still using the Sennelier VT. It seems to me to be the real stuff and I like knowing what I am using.
Hope that helps someone.