I've been using Golden Airbrush medium for a while and
really like it.
http://www.#^^^^&blick.com/products/go...FdNahgods9QP9Q
Createx also makes a retarder which can help with tip build up.
I have also used the 4011 auto air reducer with good results.
I find with createx that you have to filter it. No matter how thin you have it. I use women's knee socks (hosiery) and medicine cups. You may be may be amazed on how much better it sprays and the nice coverage you get.
I've switched to using a lot of golden and the iwata paints. They flow so much better. More pricey per ounce but they eliminate lots of issues. However last I checked, the golden paints have changed some to more transparent instead of opaque.