Might want to try a 40 weight to see if the psi stays up first. Did you put new oil in it when you got it back running? If you didn't the oil could of picked up the contaminants on the bottom of the pan causing the oil to turn black. It could also be causing your oil filter to be clogged causing your psi to be low.
If you want to find out if there's one or 2 sensors, trace the wiring or just look for the oil pressure sensors. If theres only one check to make sure the wires are hooked up good.