Hello William
According to your description, your truck is 392 series with the VW BEF engine.
When engine lubricating oil pressure is too
low, this indicator lights up, and a buzzer
sounds.
If in addition the text field shows
the text "Oil" with a double arrow pointing
downwards, the engine oil level is too low.
The possible problems:
- Insufficient oil in the crankcase
- Engine too hot
- Incorrect oil used
- Internal leakage in the lubricating oil circuit
- Top up with motor oil.

Check all the issues. Should be simple.