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I would say one of two things; Valve seals or sticky rings. Valve seals pretty typically do this in gassers. Dunno what they look like in a 7.3 IDI. The other thing is stuck rings. Carbon builds up in the ring grooves and plugs the little ports that go to the oil control rings. Sometimes they'll get stuck just from sitting in the same spot enough, but if you heat up the engine a little or rev it high, the pistons warm up just a little more (aluminum expands more than iron) and pushes the rings into the ring grooves sticking them in the carbon.
The fix for the rings is to do a motor flush. Follow the instructions carefully, if you load up the engine with solvent in the case, you'll be buying a new one. But it cleans all the oil ports out really well, and very frequently unsticks the rings. Expect oil pressure to rise slightly, too. I do this about every 100k if nothing else but to keep the oil flowing through the rods and pistons. After doing teardowns after a flush, I do a flush before tearing ANY engine down.
As to valve seals, they can be done in place but it's a pain. I'd just burn the oil, personally. But you can TDC firing each piston and pull the rockers, spring and keepers and slip a new one on each valve. Just make SURE your piston is TDC, or you can drop a valve!