I refer to the valves as LS or Magnum as the OEM source. I am using the Melling 5.3 LS valves in my Ford 289.
As to stem diameter, the difference between 5/16" and 8mm is only about 0.002". A 5/16" ID guide can be installed and the option to run 5/16" or 8mm stems is open. All that needs to be done is hone the guides to proper clearance.
I think a number of 4 valve engines are running 5mm valve stems.