On a B18 or B20, there is a small (5/8") square headed brass nozzle inside the timing cover. Its purpose is to spray oil on the gear teeth through a tiny hole. Sometimes it gets removed and not reinstalled (or broken off by accident) during an engine rebuild. If this happens a large amount of oil wiil pour out onto the gears, which is not a problem in and of itself, but it will also greatly reduce, if not completely eliminate, oil flow further down the line, as all the pressure is lost through the big hole where the nozzle is supposed to be. I haven't looked at a diagram of the oil flow for one of these engines in a long time, but I suspect it goes from the front cam bearing (near the oil squirter) to the middle one, which in turn feeds the rocker shaft, as already described.

As far apart as you already have it, removing the timing cover to have a look is very little additional work.