Unless you were running Mobil 1 5w30, I wouldn't switch to it. Over 75k, on a Conventional oil, I suggest Castrol GTX 10w-30. No, 10w40 or 20w-50, 10w-30.
If you have been running a 5w-30 syn, you can either leave it alone regardless of the mileage or switch to 10w-30 syn. I wouldn't go from a conventional oil to a syn with 100k on it.
Go pick up some Valvoline Max Life. They reccomend using it after 75,000 miles but I started using it when I hit 90,000 and been getting about 40 more miles per tank ever since.