i'm thinking about doing a full flush and fill with mobil 1 synthetic in my 00 se...i currently have 49k so the tranny is still cover under 60k powertrain warranty...my question is does switching to synthetic voild warranty?

How would they know if the fluid was synthetic or not?

according to the owners manual your good to use the synthetic.

It will not void your warranty as long as it satisfies Nissan recommendations, i.e. Dextron III.