When you installed this temporary guage do you know that the guage is accurate? Don't rule out a pinched guage tube or crud in any fitting or block.

If you drop the pan be sure to check out the oil pump before going further into the engine.

Tom Langdon has a good article about these oil pumps.

http://stoveboltengineco.com/2010/09/prepairing-the-235261-oil-pump/