I had opposite, too much pressure and a warning light, on another aircraft type.
Try this, I downloaded AirEd - or you can use another .air file editor - found the Oil Pressure setting and made the adjustment there.
In the feelthere Caravan here's the settings for your reference;
Engine Oil Pressure
- Oil Pressure Scale Factor=0.850000
- Oil Pressure Limit 144psi=864.000000
- Oil Pressure Rate of Change=0.8000000
On this vid you can see the Oil Press light extinguish right after the engine has stablised after start sequence. (1m 13s)
Found it. Its the Oil Pressure Scale Factor in the .air needs changing, as above.
I just adjusted mine back from 0.85 to 0.25 and the lights shining bright!