I wouldn't use the 10w30 by itself. You might get away with 'blending' it with 6+ qts of 0w40 or 5w40, depending on what you can find in your area. Either one of these is recommended for the XFire. (Note: The 5w40 ESP Mobil 1 is actually better than the normal 0w40 Mobil 1.) But even doing this is risky since M-B specifically states that Mobil 1 10w30 doesn't meet the requirements for high temperature high shear strength needed for the 3.2 liter V6. (see attached)

Another possibility is take the 10w30 qts you have back to your local car care store and ask to swap them out for the correct 0w40 or 5w40 Mobil 1. It's not like you're bringing them something they can't sell. Tell them you got caught in a car upgrade and the 10w30 isn't suitable for your XFire. They most likely will let you do this. Then you're really out nothing (except maybe a restocking fee) to do the oil change.

Later,