Just wondering: how many miles do you have on the oil? It does take a long time to purge the air out after you change injectors, but if you are getting any air in the oil (like foaming from lots of miles on the oil) it will most likely never really purge. These engines really do need the right oil (think about how the injectors work, they are very hydraulic) and I've seen more than one no start powerstroke fixed with an oil change.
If it's pretty fresh don't give it a thought, but if it's been in there many moons, or it wasn't diesel specific oil, it could be your issue.