Mercedes Benz repair: 1980 300CD oil level variance, oil level gauge, oil pressure gauge

Question
The oil level gauge on my 1980 300cd registers full when I first start the car and when my foot is on the gas, but when idling the level drops to about half.  Is this symptomatic of anything other than a low oil level?  I suspect the drain plug is leaking, but I am confused by the conflicting readouts of the gauge.  

Answer
Answer:
I do not recall the 1980 300CD as having an oil level gauge, there was an oil pressure gauge, and what you describe would be normal function.

I do not trust any oil level gauge.

In the morning (engine cold), check the oil with your dip stick, if the level is OK, ignore the level gauge.