I wouldn't bother with sea-foam in that diesel, it'll do no good. I would make sure to change all the filters, and keep up on that, diesels do not like to be shorted of fuel. Also, make sure there is no water in the fuel line, diesels have water traps because the fuel is good at attracting water, and the water trap needs to be purged occasionally. Basically, check the glow plugs, make sure they all work, run some cleaner through the injectors, and NEVER fill up when the truck is putting more fuel in the ground. Diesel tanks collect a lot algae and having a semi-truck pumping fuel into the ground stirs all that crap up and it'll plug your truck faster than a bad burrito.
Oil in diesels will always be black, even 15 minutes after it was changed. Just make sure the level stays up and doesn't smell of diesel fuel and seems runny. That is bad. Otherwise, just let it be.
Those old 7.3l PowerStrokes are good motors capable of going 300,000 and more with few issues. Enjoy!