Not to be argumentative, but Mobil 1 is a synthetic which is designed for a lot longer than 5000 miles between oil changes. I wouldn't go longer than 5000 miles for warranty reasons, but I tend to think your dealership is a bit old-fashioned in their oil change recommendations.

I would just stick with Toyota's 5000 mile recommendation, especially if I were using Mobil 1. I could be cynical (my normal tendency) and say they're pushing extra oil changes. However, if they suggest getting your own parts and they'll supply the labor that's obviously not the case. However, it will save you the parts cost and time spent waiting at the dealership to the tune of 1 extra oil change every 10,000 miles.