If you are a historian, do the dig. Set up that Windows XP VM. Load that 2.5 SDK and marvel at the renderer that gave us the sweeping forests of Cyrodiil. But if you want to build a game today , use a modern engine.
Most modern users searching for the "full download" actually just want to mod Oblivion . If that is you, do not download the raw Gamebryo SDK . Instead, download Oblivion Mod Manager (OBMM) and TES4Edit . These are free, legal, and 100x more useful. gamebryo engine download full