Bad Bishop wrote:
"Superman" (1978) is an official title, at least according to IMDb. It's also what was displayed in the opening credits.
"Superman--The Movie" was only for advertising purposes.
It's branded as Superman - The Movie
in all of its home entertainment releases. That title has solidified as its official title, even if it was only initially meant to be used for advertising purposes. You're right, though, that the opening credits just say Superman