according to my alldata, the oil pans have different part numbers:
I looked them up as 1990 crx si and 93 civic si
the 90 comes back as 11200pm3020
the 93 comes back as 11200p08020
I don't know what the difference could be, but they def have different part numbers.
unless your pickup tube isn't in the oil it should be pumping the oil to the engine.
Remember that the oil pump is a positive displacement unit, it can only move what is there, also remember that the inside of an engine is just one big controlled oil leak, if your bearings are the wrong size or you have a crack in one of your main oil galleys then you won't be able to produce oil pressure.
Alright heres some history, i had my d16z6 rebuilt with pistons, eagle rods, new rings, new bearings(main and rod) the crank was polished and balanced and the cylinder walls honed. When i got it back from the shop i put it in and cranked it some, never built up oil pressure, i replaced the oil pump with one from another z6, cleaned up the pickup and replaced the pickup gasket, built a very small amount of pressure. After weeks of trying and stuff i guess i destroyed the bearings. I had the bearings replaced again. Put the motor in, still no pressure. I dropped the pan(a6 pan was the only one i had that wasnt beat up) and replaced the oil pump AGAIN with one we know for a fact works fine(came off my brothers z6 he just swapped out) Still nothing, tried crankingthe motor with the plugs out, no pressure builds up, checked oil countless times, not low. I just dont get it. Is it possible that having an A6 pan will cause this problem on a z6? I looked and they looked pretty much the same size. but could that be a problem? I will be working on it again one of these days soon, but does anyone have any ideas or suggestions? thanks for any input 
