OK, sounds like not a cylinder issue. Oil weight is not too low.
Oil brand? Looking to see if you have adequate ZDDP just in case the cam dropped a lobe. But that would likely show up in a poor compression on the effected cylinder, so probably not that.
So that goes back to a lifter having an issue and not holding oil pressure and being beat down by the spring pressure. I guess some Marvel Mystery Oil or Seafoam would not hurt at this point to see if it cleans out some speck of crud.
Again, this thinking is just from listening over internet devices which is not the best sound quality. But it sure sounds like the Crane lifter that slowly went bad over the course of 1000 miles in my son's 340 engine... which would get worse after warming up. It has the same compression level as your engine, and that seems to show up weak lifters and cylinders quickly. You could hear one exhaust side with a distinct mild 'miss' when warmed up.