On the standard wiring/sensing setup (no mods like I've written about in the past), the only way you'd get a high needle would be a fault with the cluster, a broken wire, or an uplugged cluster connector (one of the centee two specifically from experience).
The way gauges that hold thier value with the key off work is that there are two sides to the coil assembly that moves the needle. One has a fixed current running through it, and the other is variable (where the sensor comes in to play). The needle's position is determined by the relative difference between the currents flowing through both sides of the gauge's coils.
In this case, the variable side is a switch and a 20 ohm resistor (in the cluster), which switches the second side between open and 20 ohms, which provides the two low and middle positions (hence the "mechanical idiot light" designation). However, if the fixed side loses current (EDIT- likely not your problem, see below), the needle will go full scale towards the high range with ANY current flow in the variable side, as any current is more than 0. This also means that with this fault, the needle will stay high until the fixed side gets current flow to pull the needle back to 0 (or middle if there is secondary current flow).
Another possibility (which sounds more plausible reading your most recent post again) is that the 20 ohm resistor has shorted out. On the cluster it's near the oil pressure needle and marked "20Ω" on the plastic circuit board. If it has shorted, either cut and solder a new one (you'll have to order it from a electronics part store/supplier), or
take the opportunity to swap to an older-style variable sending unit.
TL,DR: Assuming stock wiring, if the needle goes all the way to/past high, you either have an issue with the cluster or the wiring/connections going to the cluster, not the sending unit or actual oil pressure.
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