Biodiesel is also known as an excellent cleaning agent. If this was your first tank of B20, it's also probable that it's the truck's first tank, and any kind of sediment or sludge in the tank would have been picked up and deposited in the filters. I don't know if the 7.3's have fuel pressure sensors, but that could be what's tagging the light.

If it's been more than a couple thousand miles since you changed the filters, you might change them again. I've had the filters on my Series 60 clog badly after only 1 load of fuel (within about 200 miles, no less).

-blaine