I tried making a You-Tube video showing one of my oil system tests.
This is the assembled engine, except one pair of lifters are removed to view the oil flow through the small (0.030"-0.035") hole in the lifter bushings.
I was using a slow (600 rpm max) battery powered hand drill to turn the oil pump, 64 psi with these lifters removed. Nice to know oil pressure exists even with lifters removed.
Plenty of flow though that little hole !

YouTube LINK to oiling test

I also placed a set of pushrods in the lifters (no rocker arms) to make sure it pumped oil to the rockers/heads. Successful, but my video is too dark for a good view. The oil flowed out the rocker end of the pushrods, but did not shoot out like in the lifter bore. There was no pressure on the pushrods except their own weight resting in the lifter cup.