By spinning the oil pump and seeing 15psi on the gauge I would think you are OK (assuming the gauge is good). Cold oil coming out of the rocker arms takes a long time when only spinning the pump. I saw the same thing with my 235. Once you get the engine running and the oil warmed you will see alot more flow from the rocker arms.

You won't necessarily see oil in a translucent plastic line unless you have purged the air out. This is a dead-end pipe. When the oil pushes on the air it will compress and allow the gauge to read. You can purge the line by pressurizing the oil system and loosening the line right before it goes into the gauge until oil comes up the line. Just like bleeding air from brake lines - except an oil gauge will work correctly with air in the line.

Why the "new" dash gauge doesn't register is another matter. You could test by setting the regulator on your air compressor to 15 psi and pressurizing the gauge to see if it responds. Maybe you have a crushed farrell on the plastic line?

Keep checking it'll come to you.

Paul