Long crank to build oil pressure then starts. What could be the cause?

Well, I can understand your frustration. There are so many service bulletins and things that can occur with the 6.0L Diesel engine Ford and Navistar partnered on. I can't even possibly remember all the things that could be at issue, but one item I do recall is the STC branch tube from the oil pump that can be breached and cause aeration, requiring excessive crank time. There's NOTHING easy and cheap on these engines, and this repair is one such thing. However, if you find yourself a good, highly trained Powerstroke 6.0L technician, you'll have an amazing resource as you continue to own and maintain your 6.0L Diesel. That would be what I recommend -- find that one person in Tennessee that is well experienced. It makes a world of difference. Good luck.