I have used the Testor's acrylics a lot with my air brush. Not one time did I thin them. I just made sure that they were mixed up well, put them in the air brush and painted away. I had my pressure set near 25 psi, but I have had them work fine at a lower psi.
The results were not bad, but since I started using Tamiya, I won't go back to Testors. The Tamiya is just better in my opinion.