Hi Greg,
Shielding gases used to weld austenitic stainless steels include argon, argon/oxygen, argon/helium, and helium/argon/carbon dioxide. Argon/oxygen mixtures result in some oxidation in the weld pool that promotes better wetting action and improved arc stability than that obtained with pure argon. Argon/1% oxygen is very popular for welding with spray transfer. Helium/argon mixtures with 2-3% carbon dioxide additive are frequently used to weld stainless steels with short circuiting transfer. Helium additions to argon widen the penetration profile of the weld bead. As Niekie indicated, the addition of too much carbon dioxide will cause a loss of silicon and manganese, and can lead to an increase in carbon content in low grades of stainless. Carbon absorption could impair the corrosion resistance of the weld.
Here's a link to an interesting article in The Fabricator Magazine about selecting a shielding gas for stainless steel.
http://www2.thefabricator.com/Articles/Fabricating_Article.cfm?ID=903