I also saw American in Paris yesterday. 35 mm is my guess. I couldn't help but wonder how much better it would look if it was a clean re-master. Also I was irritated at the projectionist for not setting up the projector for the film ratio (wider than old analog TV, narrower than HD TV, and way narrower than any modern movie). The tops of heads kept getting chopped off, because he set up the projector to fill up the screen horizontally.

Big ol' nerd, just watch the damn movie.