So I picked up a 1975 Datsun 280z with a 350 swap. The car sat in duds garage for around 15 years. Y exactly I'm not sure, any way the engine fired with no issues, but once it warms up the oil pressure drops drastically low 5psi. But while in the throttle the pressure still builds and seems to hold. Is this normal? Or do I have an issue?
I have been told a few different things,
1) oil pickup might be clogged
2) oil pump is failing
3) the piston rings are bad
4) the gauge is bad, but it's a direct reading gauge.

Any advice? I plan on pulling the engine and taking it one theory ata time if I can get a solid answer.

Thanks for the help.