You probably have a leaking fuel pressure regulator (FPR), which is part of the CMFI assembly. You can buy just the pressure regulator on eBay for on the order of $30. To test to see if you have a leaking CMFI assembly, attach your fuel pressure gage, then turn on the key to energize the fuel pump, which will run for about two seconds, if the van is not started. Now turn off the key, and see if the pressure holds for a minute. After one minute the pressure should be at least 45 PSI, if not, you have a leak.
The fuel pressure regulator is located on the passenger side of the CMFI assembly. You can remove the idle air control valve located on the top of the manifold to have a way of peeking inside the manifold, with the aid of a small mirror and light, without having to first remove the upper manifold.
I had a similar problem to the one you described, except in my case the leak was so bad that the three cylinders on the passenger side of the vehicle were not firing, because the mixture was so rich. And because the leak was so large, gasoline was being forced around the rings into the oil, which resulted in the low oil pressure reading that you mentioned.
There much more info at the web site I've included at link to, and also picture of my leaking FPR there.