I have the issues you're referring to, but I have too many comics to flip through to find them. It was in the 80's somewhere around Superman 20, I believe. It was a mini-story that ran with Superman on Apokolips, believing himself to be a son of Darkseid. He had a beard in that story.
There was also a story by Alan Moore in which Superman lived in a mountain and was older. I believe it was right at the same time as Infinite Crisis ended and the new continuity began. I think it was Alan Moore... But it's definitely Superman in the future.