Back in the days of non detergent oil or oil with very little detergent, it was common for small oil passages to clog up. Chev, as well as Y-blocks were notorious for the rockers not getting enough oil and they would quite often get so dry that they would squeak. The common fix for a Y-block was to run an extra oil line to the rockers and the common fix for Chev was to pinch the overflow line in hopes of keeping enough oil flowing through the rocker shaft to prevent clogging of the oiler holes in the shaft. For the most part, both methods worked in their day.