Use lossless export or maybe visually lossless codec, like Lagarith or UT. Anything else is lossy.
Why would you do that? Common wisdom is to finish a project in the version you started with.
If all you need for further editing is the contents of your Timeline,
then you should follow Harm's sage advice.
Lagarith Lossless Video Codec
Ut Video Codec Suite
But, if you will need all of your source media files to continue the edit,
you should consider using the Project Manager.
By using the Project Manager, you will be continuing the edit
with the original media files and there will be no quality loss.
Be aware that once you have edited the project in CS6,
it will be problematic (but not impossible) to return to edit in CS5.5.