I currently have my setup for multichannel audio as follows..Under general setup the DVD audio mode is DVD-Audio and the SACD priority is Multi-Channel. In the speaker set-up page I have down mix set to 5.1, all speakers set to large and subwoofer on. Under audio setup I have digital output set to raw, lpcm rate set to 48khz and hdmi set to off. My question is this...I have some SACDS in 2 channel and some in surround format. I am assuming that with the setup I have now that any multichannel sacd will play fine. If I play a stereo SACD do I have to go back into the Oppo menu and change downmix to stereo or is there a way for it to automatically to choose the correct format?

You should leave the Downmix at 5.1 - just go to the Pro Logic II config and set "Pro Logic II" off. This way, stereo SACD will still pass as stereo.