Something's wrong here..... I've looked at the wiring diagram you supplied at No. 32, and it seems to have a + and a - connection to the battery, as well as a connection to the oil pressure sensor. At the risk of sounding silly, have you run the engine with the oil pressure sensor in place and wired up? There are some gauges which register "high" until the sensor, be it temperature, oil pressure, voltage etc. sends a message to the gauge. It's a sort of "fail safe" to indicate there's a problem in the wiring when this happens. Many eastern European cars had this type of gauge in the 60s & 70s, Wartburgs and early Skoda 100MBs, to name but two I know of.
If you HAVE wired the gauge up, connected the sensor, started the engine and STILL have a maximum reading, I suggest you disconnect the lead from the "-" terminal on the gauge to the earth on the battery and see what reading (if any) you obtain then.
Another question: what range of values does the gauge have? They're usually 0-10 BAR, although sometimes 0-5 BAR.