I have a 17 year old BMW. To clean your winshield of anything, I
have used "Bon Ami" cleaning powder.
This is NOT sold in any store that I know of. Don't mistake this for Comit or anyrthing like that. This is an extreemly fine powder.
This is a very old formula. The only company that sells it that I know of is:
Faultless Starch / Bon Ami Company.
Kansas City, Missouri 64101-1200
I don't use it to often. It takes off all the road scum that mess up your winshield wiper blades. Try it when you install new blades.
But don't make a habit of using it. I also use "Stoner Invisable Glass" spray for winshields. I have tryed a few and this seems to work the best for keeping the road scun off.
Good luck
Walt
PS
we use it for every day cleaning also. ( Bon Ami )