i also have buildup on my windshield. I have not gotten around to cleaning it, but I am fairly certain it is an oily buildup, and will appear clear when I do finally clean it. It is really only noticeable when light shines through the windshield, such as direct sunlight, or passing headlights.
I use a heavy VG mix at 5.5 volts, and with that, I can fill the car with so much vapor that I can barely see out, if I try.