Kurt, T.M. Maple summed up best IMHO how to handle Superman's values about honesty and not killing.
I question the claim that Superman had an oath "never to tell a lie" -- or if he did, he sure broke it a lot.
Grabbing a couple of pre-Crisis issues at random, I find that in #295, he returns to the GBS studio as Clark after foiling some danger as Superman, and claims that he had to leave because the tension of the unfolding crisis had upset his digestion.
In #336, he stops a runaway truck as Clark, then melts the asphalt beneath it with his heat vision and claims that the truck must have sank into the street because the asphalt hadn't hardened yet, slowing it enough for him to stop it. Later that same issue, as Superman, he "returns" a pendant to Rose Forrest, except that it's a non-dangerous duplicate of the pendant, which means he's not only lying when he says he's returning it, but he's a thief as well.
I'm sure we can all remember many times he's claimed illness as Clark, or that he caught a train back to the city when he actually flew off to outer space, or that he got Clark back to a hotel or other safe spot before appearing on the scene, and on and on.
His lies aren't harmful -- well, except when he lies to villains, and the villains wind up being harmed -- but they are lies.
kdb