These are low pressure engines. 15-40 range is normal (depending on speed, or RPM's). If you tried to jack it up around 60-80, you'd spew oil out of every possible orifice.
I'm sure others will chime in, but the manual states pretty low pressure.

Brian,
40 psi is a pretty common oil pressure to see spec'd in full-pressure lubricate engines of that era. I wouldn't call it low pressure......just ordinary pressure. There's no point in increasing it any as there is no gain, and any increase results in higher torque required to drive the pump and increased wear on the drive gear, however, I'm curious as to why you are seeing only 30 at the high end. Also, why you are using a 20-50 oil? Heavier isn't better so it implies that the previous owner may have been attempting to raise the pressure by using heavier oil (assuming it had the 20-50 in it when you got it). Being an habitual cynic, I'm always suspicious of the term "just rebuilt". Was he/she specific as to what the "rebuild" included? Either the engine has some pretty loose bearing clearances or the oil pressure regulator isn't performing up to snuff. A weak or broken spring in that device will limit pressure to something much lower than factory spec. If it comes up to 30 fairly quickly but holds that pressure, unchanged as rpm increases, I'd be suspecting the spring. It's easy to check.......just grab your 1-1/8 box wrench and remove the regulator bore plug, just behind and below the distributor. If the spring is broken, it will be obvious, but if it's weak/collapsed it won't be so easy to tell. Dang....I should have measured a good one before writing this so I could tell you what it should be.
Another clue:....if you are seeing 20 psi at hot idle, that's pretty normal and implies reasonable bearing clearances. This also points to the regulator spring as the possible culprit.
Frank

Thanks Frank,
I am seeing 20 PSI at idle when hot. It comes up to 30 PSI when cold. I have some receipts for the engine rebuild to support the previous owner's statement. The car is currently indicating 70,000 miles (reasonably well documented so believeable). The reason for the rebuild was apparently due to a bad score in one cylinder and I have receipts for new pistons, cam chain, sprocket etc. I believe it did have new bearings - not sure if I have receipts but the previous owner did say the crank was re ground. The previous owner used 20/50 so I continued with the same spec oil. Thanks for the suggestion to look at the oil pressure relief valve, I will remove it at the weekend. Is it possible to get a new spring, are they remanufactured ?

Thank you,
Brian.

Brian,
If the crank was re-ground, then it's virtually a given that the bearing are new. 20 @ hot idle is within reason.
Do you have a canister-type bypass oil filter on this engine?
Frank

Hi Frank,
the oil filter is an original style paper element inside a separate steel container mounted on the block near the distributor.
Brian.

I have just found the receipt showing in addition to new pistons and rings, a set of main bearings (-30 thou), rod bearings (-30 thou), oil pump kit, and cam bearings.

Brian,
Those are good indications that the job was done well, however, it is often true that the regulator is simply re-assembled with whatever spring was in it prior to the rebuild. It's even possible that the rebuild was done in an attempt to correct anemic oil pressure but didn't have any effect since it was the spring to begin with......just a thought.
I have determined spring force specs at the various lengths that are significant when installed, i.e., Installed length (relaxed, engine off); @ crack point of the bypass filter port; @ crack point of the regulator relief port (engine running at full regulated pressure). I have also identified a commercially available replacement spring (not a hardware store item) but I'll need some time to locate that info or repeat the process to determine those specs. I'd put it in a computer but then it would be lost forever.
F

Frank,
thanks again for all the information. I am inclined to try to replace the spring. If you are able to locate thet details for a spring I would be very grateful. I have not yet removed the valve. Is it a ball bearing type or is it a plunger type? Is there any tendency for them to stick? I have had the plunger type stick in other engines and have had to do careful cleaning up to get them working properly.
Regards, Brian.

Brian,
It is of the cylindrical plunger type. The bore plug and spring will tend to launch when the plug is backed off. You are right that the plunger could be jammed with debris from the "break-in" period or whatever. That's the most important time to have a full-flow oil filtration system in place.
(Shameless plug, I realize).
Frank

As frank has mentioned. While you have the spring out, finish removing the plunger. I have seen many,many times, the oil relief was never taken out during the rebuild,hot tank or even checked. If there is debris in the bottom of the plunger, it will let oil by-pass pressure. This really needs to be checked closely. A bright light, and inspection mirror work well. To remove the plunger, it should be easily pulled with a small magnet. If it doesn't come out easily,sometimes you can carefully use a pair of snap-ring type pliers to stick inside the plunger and pull it out. Make sure that you are in the center of the plunger and not on the bore of the hole. This will only cause further problems. Hope this helps.

If a stuck plunger is the cause, it's because it is stuck in the open (relieving) position. Oil pressure pushes it to that position but the spring isn't strong enough to return it so it fails to control pressure. Higher pressure at Cold start-up probably pushes it beyond where it would be after the oil thins, further aggravating the low pressure condition.
It is likely true that the plunger isn't always removed and properly cleaned in some "rebuilds" partly because some don't even realize it's there and there often seems to be a smoke-and-mirrors attitude about what it does.
If an engine has suffered a catastrophic mechanical failure, the likelyhood is pretty high that debris will jam the regulator plunger.
Frank

I found the plunger was moving freely at the far end of its travel and was able to move it around with my finger, but I could not remove it as it stuck halfway out. Then I used an 8mm rawlbolt with two nuts as spacers and managed to extract the plunger using the rawlbolt. I did clean out the valve bore and I cleaned up the plunger with a Scotchpad. I did not find any debris in the bore. It is all back together again now, ready for a test drive.

I would add one more thought to the mix. There is a small expansion plug in the block behind the timing gear. it terminates the oil gallery that lubes the lifters. If that plug was removed during the block cleaning, it may not be replaced. That will cause a low pressure after warm up. To determine if it is in place, you can adapt an air line to the oil pressure port between the valve covers. Remove the front valve cover and supply air pressure. If you hear air flow in the front of the valve chamber coming from the timing cover area.., the plug is missing. After doing this..make sure you crank the engine over for a while with the ignition switch off so you replenish the oil that was forced out by the air pressure. Don't ask me how I know this.. LOL

"Stuck half way out" could easily be the cause. What position was it in when you removed the plug? Was it half-way out then? If so, that would explain the low pressure since the plunger wasn't being pushed back in by the spring, thus leaving a big hole, prematurely exhausting the needed oil volume to the reservoir instead of contributing to lubrication.
Not to pick Dave 's post apart, but the plug is at the end of the main oil gallery. There are small diameter holes that intersect the gallery at each lifter bore providing pressurized oil to those parts. If that plug was to be omitted, I'd guess you would never achieve more than a few psi of oil pressure under any conditions but I have not actually seen it.
Frank