Given the tone of some of the other emails, I'll offer another suggestion.
Does the pressure fluctuate at all at the lower RPMs?
If it does not, I'd suspect something with the oil pump, perhaps an obstructed suction screen that runs out of capacity at higher RPMs (requiring more volume).
I'd also check with a pressure gauge in various locations- like right out of the block, and then after the fuel shut-off. From the factory, your 620 had a fuel shut-off that is fed by the oil gauge line. If the pressure fails, it is supposed to shut off the fuel. Wierd things can happen with these after nearly 60 years.
Don't worry about the rude guy. He is wasting his time here, too.