Fabio,
White spirit is actually a refined form of turpentine. You need white spirit to thin Testors Glosscote, Satincote, and Dullcote, as well as X-tra Color paints. I've tried normal turpentine on these paints but the X-tra color tended to go a little grainy and the Glosscote didn't really dry.
I didn't have these problems with these products when I used white spirit as a thinner.
Cheers,
LeeTree