Bill, I have one of these guys for small painting projects that I do not want to bring out the big rig.
The paint has to be thinned or the power painter will spit and sling like you would never believe.
With the original power painter came a divise that you use to measure the proper viscosity of the paint so that it will mist properly.
The tool is very rudimentary, but functional. You dip this dowel into the paint and you measure the time it takes for the paint to drain down the sides of the guage. If it is too slow, you add water to speed up the fall.
Bill, most paints, especially latex do state not to dilute, but the amount that you will dilute will be rather small. You really do not have a choice, because the machine will not mist thick paint anyway.
I would go ahead and dilute an amount and spray it. You should not have any problems.
Viscosity: Glidden paint is like mayo. its very thick. No way will this spray in your rig. you want to be somewher a round the viscosity of 30 wt oil. It will take a while, but with a big piece of cardboard and a calm day, you can experiment with the viscosity of paint to determine the best mist.
Remember, the sprayer must mist or atomize the paint so that you get a nice fine spray. Spitting, and slinging will not get the job done, only a good mess.