I have found that some of the gunk people spray on their seats and dashboards to make them look "nice and shiny" contains volatiles that stick to interior glass surfaces when the stuff evaporates. No matter how many times you remove it, the the oily film will reappear the next time the car interior gets hot, so if you are using any type of interior spray, stop using it and see what happens.
If this is what is causing the problem, it will go away by itself eventually, but in the meantime, try covering the dashboard with some impermeable material that will stop the volatile material from getting onto the inside of the windscreen.
In one case I remember, my client had to replace the dashboard because the previous owner of the car had used so much dashboard spray over a long period that the entire foam structure had become saturated with the stuff.