AU Oil Gauge (Fairmont Series I)

[Hords]

G'day all,
Just wondering if anyone experiences the same or had the "problem" rectified.
My missus had the AU I for about 2½ years. She's now gone a BA Ghia 8, so I've taken over ownership and want to tidy a few things up.
On start up the oil gauge will take around 8 - 10 seconds to climb from low to high (redline) and then stay there. Occasionally, but not always it will drop to about two thirds when in idle. This is the only to reading variables I get. I would assume the gauge should be reading around ½ way under normal operation and then increase a bit under revs.
Any info would be appreciated.
Thanks