On white or light colored cars, I use Tamiya X-66 grey diluted in Tamiya thinners to create a wash, on dark colored cars, I use Tamiya X-18 Satin Black diluted in Tamiya thinnersro create the wash.
If you go to
www.tamiya.com and look on the English site, they have a tips section at the bottow left of the screen which explains how to black-wash a car/model.
Ray