I never did have a smeary screen, but the 159 has auto intermittent wipers that go absolutely crazy with a greasy windscreen. My first few months with the car I tried a variety of cleaners before finding what works. No doubt there are other solutions, but what worked for me was:
Just the once: a heavy-duty de-grease with white spirit. Then a polish with Turtle Wax Intensive Cream Glass Polish, which left the glass noticeably smoother to the touch. The next couple of car washes I followed with Tesco Glass Cleaner to remove any wax or detergent residue left on the screen, and the auto wipers gradually became more sensible.
About 6 weeks later I gave it one more go with the Glass Polish, and a squirt with the Glass Cleaner whenever the wipers became erratic, as well as after every wash.
Three months down the road the auto wipers are reliably well behaved, suggesting that the screen is now clean enough.
That works for me. If Turtle Wax and Tesco are not your style, no doubt your preferred brands could do as well if not better.
Last edited by ChrisH77; 09-12-15 at 19:46.