When I bought my wagon from a guy in NJ, he told me "the motor was rebuilt by the most reputable shop in NJ."
One day, I was driving it and all of a sudden the oil pressure dropped. I shut it down. After a couple of minutes, It started and ran normal.
When I got it home I pulled the rockers and noticed 2 of the lifters had holes in the center where the rocker rides. The ball on the rocker was worn and had a small bump from the hole causing the rocker to shift and oil pressure to drop. Evidently, they substituted a lifter from an OHV motor which uses that hole to lube the rocker for the Opel one. The lifter dimensions were the same with the only difference being the hole in the center.
There is a GM lifter somewhere out there that we may be able use by swapping out the center piece.