Good question.
We've discussed this in depth, I wish some other old timer here could dig up the thread. Basically it has to do with an oil feed restriction in the block, later years were redesigned with better oiling supply, so the dump tube returns excess lube from the top end before it travels down the rocker shafts.
Its a interesting story about a removable oil line, hydraulic lifters, calibrated orifices and a special "grooved headbolt" that is listed in the books but has never been found in a motor.
perhaps somebody here feels like typing the backstory.....
