I have seen many 216s and 235s look just like the images in starkweatherr's album. It is amazing that they run as well as they do. The cause is almost always due to blockage in the oil passages to the rocker arms. As 31,32,33 suggests, remove, disassemble, and clean the rocker arm-shaft assembly. It would also be a good idea to remove the pushrod cover and clean out the small tube that runs from the aft end of the pushrod gallery to the the fitting near the upper-center of the block , like this . That is how the oil gets to the rocker arm assembly. After reassembly and before starting the engine, it would be a good to drive the oil pump ( with a screwdriver bit in a drill?) until you see oil coming through all of the rocker arms.

Hoyt