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My engine seems to run with a low oil pressure at speed and when cold, it never indicates much over 40 psi which I think is just over half way up the guage. At idle it doesnt drop much under 30 either. I dont know what oil is in it and may change it so that I know it has a suitable classic grade 20 - 50.
The engine was rebuilt by a professional shop that has experience in these engines in 2000 and the car has only 500 miles since (approx).
The oil filter was "cut apart and cleaned and converted internally to a spin on filter", but leaks slightly at its joint around the circumferance in the middle of the filter unit.
Engine runs well generally.
I have not checked the regulator.
I would have expected it to to indicate at max pressure on start up and at speed.

Those readings (assuming the gauge is close to correct) of 30 to 40 psi are just about right for that engine.

My repair manual only goes back to 1940 but it says a Super 8 should have 50 PSI @ 30 MPH. Naturally it will be lower at idle especially hot. I would expect 15 or 20 PSI at idle on a rebuilt motor.

Try a good mechanical oil pressure gauge if you want to be sure. Your gauge may not be quite accurate after all these years.

My 1935-53 Vintage Car Motor manual shows a 1939 Super Eight with the 320" engine at 60 p.s.i. @ 30 mph. The 1940 Super Eight 356" engine shows 50 p.s.i. @ 30mph. When oil is hot my '47 runs at about 40 p.s.i. at anything much above idle and about 15 @ idle using a high quality mechanical gauge at the valve lifter oil gallery port. The dash gauge is a bit more optimistic. I wouldn't be worried if you are getting 40 p.s.i. while running above idle.

My engine seems to run with a low oil pressure at speed and when cold, it never indicates much over 40 psi which I think is just over half way up the guage. At idle it doesnt drop much under 30 either. I dont know what oil is in it and may change it so that I know it has a suitable classic grade 20 - 50.

My 320 engine, reasonably fresh rebuild with babbitt mains and rods, idles hot at 20 lbs +/-, runs at hot road speed at 45-50, and I've set the pressure relief valve to cut off the maximum at about 55 - seen too many ruptured oil filter canisters from higher cold pressures. I run straight SAE 30 detergent, daylight temps in the summer here rarely into the 90s.

If yours never get above 40, I'd think perhaps you'll want to adjust your pressure relief valve. If in fact the cold idle pressure does increase above the 40 you now see with more tension on the plunger spring, I'd set to max to about 55-60 but no more. You might also want to check the accuracy of your gauge.

If you can't get above 40 psi with a cold engine and you've checked the accuracy of the gauge, I'd guess something internal isn't entirely up to snuff but I'd also speculate that you have enough pressure for safe engine performance.

G'day and to PackardInfo. And for including your '39 1703 and '36 120B in the Owner Registry. See you're from Edinburgh, where my paternal grandparents originated from and, where my daughter received her marriage proposal in the grounds of Edinburgh castle on Christmas Eve some years ago.