Lower contrast grades seem to last longer. I have some Oriental Seagull Grade 1 from the late 80's (the original Seagull, that Adams and Brett Weston used - the stuff in the silver package) that still prints beautifully. I also have some grade 4 of the same vintage which has lost a lot of its contrast punch.

Azo seems to last just about forever, although again the higher constrasts (3 and 4) lose a little bit of contrast over the years. Just a little. I've never kept any other paper for longer than 5 years or so.

If your Oriental paper is the original stuff from the 80s, cherish that paper. They really don't make 'em like that any more. It's really beautiful paper.