Having tried many different things over the years the only thing that I can recommend 100 per cent is jewellers rouge, it's a powder that jewellers use to get scratches out of jewellery its also used to get small scratches on windows out, sparingly use on a damp cloth and gently clean, do not press too hard, and ensure the cloth is allways damp, you can see the grease come off as you use, after cleaning just buff up window with a dry cloth.it can be purchased on e bay at a reasonable price. It works excellent I use it about once evert two months as I hate greasy windows in my car.