Did they give you a hot idle pressure value? If the pressure is above 4 psi at hot idle, the instrument cluster is probably the only thing left to replace. If the cluster is a CCN module, it may not be plug n' play. A used or an exchange cluster would have to be configured to your VIN in order to play nice with the other modules.
The gauges are designed to have a wide middle range, so they really aren't accurate, just more assuring to the operator.
70# @ 3K RPM is excellent oil pressure. The cluster is serviced as an assembly, not individual gauges. A 2004 (AN-body) is old enough not to be a CCN.
Does the oil pressure gauge do a full sweep with the cluster self-test? Hold the trip odo button in & roll the key to 'run', then release the button.
The scan tool (DRBIII or equivalent) can step the gauges through their calibration points, but I think the self-test also does this.
The odometer mileage is stored in the cluster itself. The mileage displayed would stay in the cluster. If you wanted to adjust this mileage, it would need to be done at an authorized factory electronic repair depot.
Depending on mileage fraud laws in your area, you may need a shop repair order written & the shop's handling of this repair for use of a legal mileage adjustment document.
I was thinking last night that since the cluster gauge driver can position the gauge pointer at the correct points during the self-test, that the cluster itself may be OK?
I don't want to start over-thinking this, but another possibility is in the PCM. The oil pressure information goes directly to the PCM and from there the 'interpreted' oil pressure value (now digital information) is sent over the PCI bus to the cluster.
It is broadcast across the PCI bus to all or any modules that need to know, isn't a direct wire from the sensor to the gauge like in the old days.
I think that the self-test is also performing a gauge calibration?
This would explain why it working normally now.
Heavier motor oil viscosities will raise oil pressure, but increase internal viscous drag and the penetrating characteristics of lighter motor oils. It may be OK in higher temperatures, but you want to return to lighter viscosities in cold weather.