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 15w40 was made for Ford by Phillips-Conoco and was a decent oil. I used the buy the 5 Quart Jugs at WM (3 Jugs for an oil change) along with the Factor Motor Craft Oil Filter (FL-1995) at WM.It's been a while but if I am not mistaken the oil and oil filter was about $50 or so. The oil filter was pretty large and easy to remove/install. I would loosen it slightly then take a steel punch and poke a hole in the bottom of the filter and let the filter drain into the waste container to avoid any mess before removing the old oil filter as it held a fair amount of oil.I changed the oil and oil filter every 5000 miles.
Reading about your oil pan replacement, I remember years ago, there was some type of service bulletin regarding oil pan rust on the 7.3's. The dealer sold a can of a coating for the oil pans to combat this issue. I can't remember the part number but it was in a can and it reminded me of liquid tar.
The procedure was pretty simple. I remember lightly sanding down the oil pan (not removing it) than taking a paint brush and coating the oil pan. I placed two coats on to be safe, it dried quickly. This stuff worked, never had a rust issue regarding the oil pan.
I would hang on to your Excursion as long as possible, the 7.3 is still a sought out engine for it's quality, power and reliability. When I had my 7.3, I remember many of the 6.0 folks liking my 7.3.
In my current truck (2011 Ram 3500 4x4) I have the Cummins 6.7. I only run Shell Rotella T6 5W40 year round. The Cummins seems happy. The manual states 7500 or 6 months for oil changes. I change the oil every 5000 or 6 months because of the emissions on the truck. The oil gets black real fast (soot).