New to me Barracuda with Mopar Crate 390hp low oil pressure

I changed the 11 year old Optima Red Top battery. When at low idle, hot engine and the electric fan on the volts are near 12 and slightly bounce up to 12.5. Oil PSI reads 2-5psi. But once the electric fan stops. Volt meter is back up to above 13 and oil psi is around 10-15. Gauge is not marked below 25. So I have to guess.

mechanical gauge is in the mail.

Looks like the prior owner wired the electric fan to the Painless wiring brand fuse box. I’ll change this to a relay and pull power directly from the battery.