Had a similar problem with a newly rebuilt engine - tried just about everything mentioned above, and it turned out the problem was the intake manifold: it was not tight. Tightened it with a torque wrench and it started right up.
Guys - I got it out . . . I'm kind of new to this forum, and I should have looked back for a similar problem first . . . thought of that after I posted.
I searched and found a post from 2010 on a broken oil pan bolt, and found an excellent suggestion: Since the bolt was not rusted in place...
While trying to replace the engine splash pan on the passenger side of the engine compartment, I snapped off one of the oil pan mounting bolts that is also used to hold the splash pan in place. It's for the front bolt on the splash pan, located right underneath the oil return pipe.
Any...