There is a rather convoluted oil circuit to get lubrication to the rocker shaft, and it can get clogged easily. Older engines with an oil line under the pushrod cover connected to the middle of the rocker shaft got their oil from the rear camshaft bearing. Later ones such as yours use internal passages in the block and an oil connector that plugs into a hole in the cylinder head.
Both systems can be bypassed by running an external oil line from the oil pressure gauge port, around the back of the head, to the 1/8" NPT port in the middle of the cylinder head on the passenger's side. Use 1/8" OD copper (NOT PLASTIC) tubing, the same type that is used for aftermarket oil pressure gauges. The small ID of the tubing controls the volume of oil flow to the rocker shaft very well.
Jerry
