My guess is that you spun a camshaft bearing. I went through the same thing with my motor. The pressure is good at first because the oil is thicker when cold. As soon as it heats up it is thinner causing less oil pressure.

To prove this, try putting in something like 20-50 or better yet Straight 60 weight. It will keep better oil pressure for longer. Depending on how bad the motor is.

My advice though. Do not drive it anymore!!! When mine spun it scored the heck out of the block. I ended up having to get another block because it would have cost more to try and fix the block then to get a used one.

Good luck.

 

Ditto here, I was just thinking the exact same thing. I think you have major engine issues, time for the 383 stroker

Ed