Cruiz,
I've had trouble finding "white spirit" - I've been told it's a refined form of turpentine. I looked everywhere for it but finally found it in a specialty paint store where they sell household paints.
Humbrol Gosscote, Satincote and Dullcote labels all say thin with White spirit, but I didn't read the fine print, and also found the Glosscote was slightly tacky after a week of drying using normal enamel thinner.
X-tracolor paints (quite good to use-and they produce some hard to find colors) also require white spirits as a thinner.
I don't think Tamiya X-20 would work - it's formulated for water based acrylics, and Model Master is designed for their enamels. People often say you should use the thinners the paint manufacturer recommends, but others have had great results using MEK, acetone, acrylic reducers and other stuff. I guess the rule is to experiment, and reject the stuff that doesn't work.
Good luck
Lee Tree