Interesting problem. I rebuilt a 300 6 for a truck project. Before I started the engine I ran the oil pump until there was oil to the top of the head. When I started it I couldn't get any oil pressure. Zero on the gauge. So I shut it down and installed a different gauge, same trouble. I'm using the pressure port on the drivers side between the distributor and the bell housing. When I removed the first gauge, there was fresh engine oil in it, like there is oil there. I've pulled the distributor twice to make sure it's engaged in the oil pump, if it wasn't I'm sure it wouldn't go down to seat properly. I also didn't get a pressure reading when I ran the pump with my drill, but I could feel the drill working harder when it started pumping oil. These are pretty good stem gauges from my shop, 0-500 psi in 10lb increments. Should I just try gauge #3? Or could it just be the style of gauges I'm using?

 

Gauge #3 worked! It's a generic gauge off one of my tractors, it just comes up in the green area. Kind of weird the stem gauges wouldn't work. I'm correcting myself here too - the first gauge was 0-500 in 20lb increments and the 2nd gauge was 0-160 in 10lb increments.

Tom