Oily film/residue

[STaX]

Try some 303 Aerospace on the dash. It might fog up again, after application, within a week or so but after that initial fog has been wiped away it shouldn't do it again. At least that's the way it is with mine.
If it still does that there could be some silicone in one of your dressings, silicone is renown for leaving a film, that's what it is designed to do, so you would need to completely degrease your dash. Strong APC or something meant for degreasing should do it but make sure you dress your plastics in something decent afterwards as the harsher chemicals can dry the plastic out.
What aerosol are you using? If it's one of the Wynns or Shield, I think they contain silicone oils.
As for the 'German theory'... I don't know too much about that. Maybe in the cars first year of life but after that....?