John had some good suggestions but before you tear anything apart:
Are you sure your pressure gauge is working ok? I suggest getting a pressure gauge (not one for the dash but a simple screw in unit to put directly in the oil galley in one of the plugs 0 - 80 psi range will work). This is a temporary condition. Just enough to check your dash gage. Also, clean out the line to the dash unit. they get clogged up.
You did not mention if you have a filter assembly. John mentions this. Assuming you do, disconnect the oil line leading to it from the oil galley and plug the galley hole. then run the engine and watch the oil pressure. If it is higher and stays higher, the fitting at the other end of the oil line at the filter cannister is probably not the correct one (doesn't have the correct reduced size outlet). If this is the case, you can fix the fitting you have, assuming it is a std. fitting with no restriction. Clean it out carefully, get 100% of the oil and debris out of the inside of the fitting. fill the outlet portion with solder. Drill a hole, about 1/32nd of inch in diam. that will closely match the original hole. The alternative is to go looking for a restriction fitting. The oil will not get hot enough to soften the solder.
The presssure relief valve seat could be leaking. You can remove it without a lot of trouble, may need to remove the fuel pump to get at it. Check the seat condition if you didn't do that when you rebuilt the engine. A mirror on a stick and flash light or a digital camera will be helpful or a small kid held up by the ankles. Pits or crud on the seal surface here will leak oil and rob oil pressure. You can also check the spring strength, there is a spec on this in the shop manual but it is probably ok. Do not add any washers under the spring. This will only increase the relief pressure which should already be about 60. Higher relief pressures won't help your low oil pressure problem and may cause other problems.
When you drive around to heat the oil, it may take at 10 miles at highway speed to get it to normal operating temp.
Posted on Aug 14, 2011, 5:09 AM from IP address 18.104.22.168