Luthor is approached by Tempus when he is still starting out. Tempus explains about Superman and how he causes Lex problems. Not all the details and not Clark = Superman. Things go bad between them and Lex's big goal becomes not wealth and power but getting even with Tempus. So knowing that Superman will appear he helps him because it will annoy Tempus. Still a criminal but knowing about Superman he cuts back and distances himself more before he appears.
I can see the later powerful Lex using his media holdings to make the idea of helpful, friendly aliens part of pop culture. And having a bullet put in Trask's head.
Lex's ego would make him tell Superman about some of this once he appears. How does Superman deal with a Lex who is even harder to prove anything about and part of the things make Superman look involved in things like murders.
