This is just my humble opinion, but many different companies make many different products so that we ourselves don't have to, it doesn't matter to me what those products are, if I had to go to the lengths I've been reading here to make something I would use myself, I wouldn't waste my time, further, I have a very understanding wife, but I think even she might just have something to say if I just wander into her kitchen and start boiling up chemical concoctions and brews and at the same time risking blowing huge holes in the ceiling or burning holes in the floor.
Having said that, I don't airbrush for profit or gain, it is merely a hobby, but if I was doing something for a customer I would want to be able to guarantee my work and I think this would only be possible by using manufacturers recommended materials, doing this would also give the me piece of mind if those materials were of poor quality through a manufacturing fault.
I want to airbrush or fly my model planes, so I would rather waste no time trying to create something that already exists, I'm not saying it's wrong to do so, it just isn't for me.
I am fortunate enough to be using a paint that is pre-thinned and I can thin more if necessary with a little water, there are mediums for this paint but for what i do I don't need any of them, the only other medium I use is Golden trans airbrush medium, this also thins and gives my transparent colors as well as almost eliminating tip dry.
In a nutshell I would rather pay an extra couple of bucks for something if it saves me some time.
This is not to criticize or offend, just a personal view.