The method shown here of feeding the LA head's rocker shaft off the oil pressure port is not to "correct" any oiling deficiency, or maybe it is. This method is done so you can use LA heads on a Magnum block that is not drilled for LA shaft oiling coming off the cam journals. There is NO oil going to the shaft in this Magnum/LA case and you have to use oil through pushrods/rockers and hope that the rockers will provide oil to the shaft. The modification shown bypasses the pushrod oiling requirement and allows use of standard LA rockers that oil through the shaft. With a standard LA block/LA head setup, there is no reason to do this external feed modification, other than you want to.