Where to start?.. Normally we put soft material with hard, like Babbitt bearings with a steel or cast iron crankshaft. Not in the case of rocker arms, they are forged and the rocker shafts are file-hard. All lifters follow the same plan.
I use stock rocker arms in both my Y-Blocks and FEs. I also modify the Y-Block setup by grinding the bottom rocker shaft slots (like FEs already have) and by pressurizing the shafts (again, like FE does). Ford engine engineers must have felt that a small drop of oil was enough on each Y-Block rocker arm. They did not think 'down the road' as dirt packed the tiny oil holes in the rocker arms because oil was then bypassed down the end tube. Well hey, they had no experience and this was their first try. When the engine is new the original setup works well.
When dirty oil is present in the arms, oil pressure effectively pushes it out keeping the holes clear.
Aftermarket rocker shafts are just as difficult to get through the stands so I know they are NOT undersized. That little groove inside the bottom of the arms simply directs oil to the holes for proper pushrod and valve stem oiling. Since valve springs keep pressure on the bottom of the arms, they naturally wear first. Groove or no groove, the arms still rotate on a sheet of oil. I prefer to re-define the groove when needed.
There was no restriction on the Y-Block shafts AND the end tube opened pressure to atmosphere. Pressurizing the shafts immediately raises oil pressure. Restricting the flow keeps more oil on that center cam bearing so it doesn't wear and close off the cam's oil groove. Again, FEs benefit from these 'factory improvements'.