I don't know what the "stuff" is on your windshield, but I had a similar problem with my 94 convertible. Since I leave for work early in the morning, around 1 am, I am acutely aware of a clean windshield due to the reflection of light from street lights an oncoming traffic. I did not have mine serviced, so I do not know how a gray film attached itself to the inside and outside of the windshield, over all of the surfaces. It was very apparent at night, but not so much during the daylight when I drove home, around 3 pm. I tried washing the windshield with dawn, ivory, Zanino wash, and all that happened was that the film moved around the windshield. Nothing came off on the wipe cloth. Tried isopropyl alcohol, then acetone, neither worked. Finally I tried Stoner's glass cleaner that I purchased from Checker. After 5 applications both inside and outside the windshield and hard rubbing, the film was removed. If Stoners hadn't worked, I was prepared to purchase Adam's glass cleaner as a last resort. Like I said, I have no idea what the film was, how it attached to both sides of the windshield, and why wiping it only moved it around. It must have been a mist that I drove through while traveling in the Twilight Zone. Anyway, good luck in cleaning the "stuff" off.
Barrett