Don & Neil,
This phenomenon that Don explains is what I am seeing with my gauge. It is the stock (1977) gauge that came with my C-30. When I give the boat gas, the oil pressure gauge goes down.
Neil, my stepfather is a retired SAE mechanic (40+ years), and worked at a few different dealerships. He commented to me that Volvo (the car company) modified their temp gauges so that they wouldn't move until there was an extreme change in coolant temp above some certain threshold because they had too many service calls at the dealerships for erratic temp readings on the gauges from them being too 'accurate'
I will also comment that on my old Honda project car I was getting low oil pressure readings at idle...like 12 PSI at traffic lights and 40 PSI or so on the road at 2,800 RPM. It had the supplied-with-the-gauge plastic tubing for the sender...I switched to a copper tube (sender unit is mounted on the firewall and not on the aluminum block for vibration reasons) and the 'new' idle oil pressure is almost 50 PSI.
I agree with Hanley...sounds like I need a new gauge on the A4
