While the answer that the OEM thinner is best may be true, it's also the most expensive. Several years ago, I discovered that DuPont 3812S enamel reducer worked every bit as well as OEM enamel thinners. I've had no problems using it with Humbrol, any of the Testors enamels or even the old Pactra Enamels (still have a few usable bottles of that around). Now some people may think the 15 to 25 bucks a gallon (depending on what shop you buy it at ) is expensive. But you get 128 fl oz and the last time I checked at my LHS, a bottle of Model Master thinner cost $2.49 for a 1 1/2 fl oz bottle. Plus even airbrushing my models, a gallon will last me a minimum of a year.