It has been a few years since I studied the oiling on a small block.
I know from working on a 265 a number of years back that all the oil to the lifters and then push rods has to pass through the rear cam bearing.
265's do it through a flat spot in the rear journal, later engines have a groove in the block all the way around the rear bearing bore, so the oil passes behind the bearing to the lifter galleys. I would think the groove in the block could also become sludged up.
Start with the push rods first though as Jerry suggests.
