oil pressure problem

I'd use a mechanical gage in place of the oil sending unit. See if that gage and the gage on your dash read the same? I didn't see a minimum spec for the four cylinder. 56 psi@3000 rpms. I did see idle spec for the 6 cylinder, oil pressure minimum, 6psi. You may need to check compression.

I checked i for sure have low oil pressure. starts out fine but as engine warms up pressure continues to drop. already checked the pump starting to pull bearings caps now

If I was going into the engine, I'd replace the pump anyway.

I replaced oil pump sending unit and all bearing. Still when warms up the gauge drops to zero engine runs fine

It sounds like the oil pump pressure spring is broken or the oil pump pick up is clogged. When you did the bearing did you look at the oil pump and pick up screen?

This guide may help as well

https://www.2carpros.com/articles/high-or-low-engine-oil-pressure

Please go over this guide and get back to us.