I am really puzzled by the oil pressure behavior in my 1984 300SD. The oil pressure is normal in cool weather. In hot weather, it goes to zero. The best example is in a 150 mile non-stop highway trip. With the outside temperature at 85 degrees, the oil pressure drops to zero at 65 mph. Then the outside temperature dropped to 65 degrees. The oil pressure pressure pegged to 3 bar at 65 mph.

I noticed that there is a correlation between the oil temperature and the outside temperature. I learned that there is a thermostat inside the oil filter housing that sends the oil to the oil cooler when the oil gets hot. This could explain the loss of oil pressure. When the oil get cool, the thermostat sends the oil directly to the filter, which explains the high oil pressure.

The engine has plently of oil. The oil pressure gauge operates smoothly and doesn't vibrate or wander around. I have 2 new o-rings on the oil return flow pipe. I want to look into this further, but I don't seem to have a metric Allen-socket that fits the oil filter housing. I have sizes 3, 4, 5, 6, 7, 8, 9, 10. I think it takes a 6.5mm.

I talked to the dealer's parts counter, and they think there is a problem with the thermostat. I think something else is wrong. I think a check-valve is not working right.

What is the problem?

Sounds like you have a faulty oil pressure gauge sending unit. The 300D models had a line running to the gauge (so failure is very uncommon) but the 300SD has an electric sending unit which can fail. I beleive it is somewhere on the rear of the oil filter housing.

That's good to hear that it is only the sender. The sender is located at the rear of the oil filter housing. It is odd that the temperature of the oil affects the sending unit and causes it to malfunction.

I am considering adding a mechanical gauge.