Walt,
I have never used the Schaffer's oil, but I do know that oil has a good quality. By using the 5W40 is smart as well. I bet the inside of your 7.3 is as clean as can be. When I had my 7.3, I used a few brands ( Delo 15w40, Rotella 15w40 and mostly the Motor Craft 15w40).
The Motor Craft...