One of a mechanic's worst fears is reassembling an engine and then having low oil pressure. Here are a few ideas.
1. If you used a lot of prelube during assembly it can rapidly clog a new filter causing the filter bypass to open which lowers the pressure. After engine reassembly I always replace the filter after the engine has idled long enough to reach operating temperature. If you have already replaced the oil filter with a quality unit having a bypass then that's probably not your problem.
2. Were you careful to engage the shaft that links the oil pump to the drive gear assembly in top rear of the block? I seem to recall that it is possible to reassemble the engine with the shaft not engaged at each end. Since there is some oil pressure the pump is actually rotating but possibly not at full speed. There is a spiral gear on the rear of the jackshaft that drives a small assembly. I think that you can check by removing the drive assembly hold down cover bolt.
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Be very careful when removing the drive as if the shaft comes up with the drive it may be difficult to align correctly to re-engage the oil pump.
3. I've learned that pieces of cassette guides can end up in the oil passage that goes to the oil pressure switch. As I recall anything above 5 psi to the switch results in a midrange oil pressure indication on the "gauge". Where did you measure the oil pressure to be 5 psi at idle and 30 psi at 3K rpm? Did you remove the oil pressure switch and connect a mechanical gauge to the port? If the path to the switch is blocked you may be able to clear the block with a wire after removing the switch.
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The primary chain plastic guide covers an opening in the "tube" that goes to the oil pressure switch. I know of a least one Forum member that found a piece of a cassette in that tube.
Once the oil has drained from the hydraulic valve lash adjusters it takes quite a while for the lash adjusters to regain pressure and the valve train to quiet down.