With the engine at operating RPM, the valve cover is full of oil vapor, all parts get mist or splash lubrication. It worked for millions of engines.
If you have it apart, a 3∕32” (.093”) “spit” hole in the rocker’s upper surface provides oil for the valves springs, stems, &c. These holes must be open and clean or the parts will not last. These holes must not connect directly to the rocker’s annular internal oil distribution groove (which may occur with mis-matched parts)..
