If your vehicle is showing greasy film building up on the inside of the windshield, here are the most likely causes and the parts you'll need to fix each one.
Chemicals and plasticizers from the dashboard and interior trim evaporate in the heat and condense on the glass. This is extremely common in newer vehicles or those frequently parked in direct sunlight.
Using e-cigarettes or smoking inside the vehicle leaves a heavy, oily vapor residue that gets circulated by the HVAC system and coats the interior glass surfaces over time.
Engine coolant is leaking from the heater core inside the dashboard. The hot coolant vaporizes into the HVAC system and condenses on the cold windshield, leaving a distinctly greasy, sweet-smelling film.
Using interior protectants (like dashboard shine) near the glass, or cleaning the windshield with dirty towels previously used for detailing, leaves a greasy chemical film that smears when wiped.
A leak in the A/C evaporator core allows refrigerant and its lubricating PAG/POE oil to escape into the ventilation system, misting a fine layer of oil onto the windshield.