If it’s windshield from inside the car it could possibly be stuff like 303 or other chemicals on dash, plastic. When it gets hot outside that stuff evaporates into your windshield as a thin greasy film. I suggest cleaning inside the windshield using rubbing alcohol only and use less chemical inside the car if you have to. For outside clean the windshield with less cleaner. The adage “less is more” also applies here. Using more cleaner creates streaks. I use one of those gas station squeegee with the scrubber side to agitate dirt and stains loose. And don’t be afraid to use different kinds of microfiber towels to get the streak out. I use a mix of high pile and low pile to finish off. Some towels create lint which you gotta deal with as well. Steel wool #000 is also amazing as an alternative to clay.