As others have mentioned, it only requires a very small volume of oil to keep the rocker arms and valve stems lubricated. If the top end is getting over-oiled, some of it will find its way down the valve stems, and there might be inadequate oiling of the connecting rod and main bearings due to pressure loss in the rocker arm shaft. There is a threaded plug in the middle of the cylinder head that will allow you to compare oil pressure between the main oil gallery on the side of the engine block, and the pressure that the cylinder head is getting. Several different designs of rocker arms and oiling methods were used over three decades of engine production, and a mismatch of parts and/or incorrect assembly can result in too much oil getting to the head. It would also be a good idea to run the engine with the valve cover removed and see if there's a lot of oil being squirted out of the rocker arm weep holes. A small trickle is all that's needed- - - -not a stream.
Jerry
