Here's the problem,
I've purchased The Shivering Isles for Oblivion long ago on a separate account while it was gold. The account is now silver but that does not take away the fact that I can download. The trick is that the account I'm downloading with was on a different xbox. I used "recover account" on my new xbox without a problem at all. Using the silver account that I just recovered, I downloaded Shivering Isles and other DLC for Oblivion. After the downloads were completed I got on my new gold account and tried to play but the DLC never loaded (but it had the Shivering Isles achievements enabled). After the DLC did not work, I went on to my silver account and played the game, and it had no problem at all. I know that DLC does work for more accounts that just one. As long the account that downloaded it is on the same xbox as the other.
Oblivion is installed to an external memory unit.