I have not tried Airbrushing with the new range of GW paints yet, but previously i have always used Tamiya's X-20A Thinner, Always found this works really well with the old range. Both the Old Range and Tamiya's XF range mix well too, so some great combinations of colours can be achieved. Only thing is make sure you are wearing a mask and in a good ventilated area (which ya should be if your airbrushing anyway! lol)

Alway found water the worst thing to thin down with when A'b'ing as paint will just clog on the end of your needle, as the paint does not atomise well enough.

Phill,