Rocker shims = NO rocker shaft oiling

The problem is the shim itself. The open end of the U is allowing the oil to pump straight out of that gap.....rather than entering the shaft where you want it go. The shim needs to have a round or oval hole so that it seals all the gaps.
Every hyd lifter I have worked with has had well over 0.100" of travel, with the vast majority going to 0.150" or more. The pre-load can be anywhere within this range. So most likely the shims are not needed if they are being used to set pre-load. The lower range of pre-load might be to reduce the effects of lifter pump up, should that occur, but the correct fix is to use stronger springs to control the valvetrain.