My take on this is the zzzzzzzzZZZZZZ noise was actually the hydraulic lifter "tappets" buzzing. At a few thousand RPM you would not hear the individual tappet clicking noise that you might hear while idling. Since he seemed to hear the noise at the same time the oil pressure was low leads me to believe the lifters were not "pumped-up" at that point in time. Also since the oil pressure gage fitting is connected to the top most oil galley on the engine is more indication that the loss of oil pressure at the "top-end" was starving those lifters of oil. Then when more oil was added everything returned to normal.

A note on longevity:

Wasn't there an advertisement, a few years ago where some car owners drained ALL the oil out of there engines, in the middle of the desert, and drive around for an hour or so? It was an advertisement for Mobile 1 Synthetic oil.

 

...hopefully nothing happened!

 

Try doing that on a 13b from the rx-7 and see what happens.

 

bwil': Did you ever hear of anyone trying that stuff 'bout driving on an empty sump pan?! I know I WOULDN'T try it, let alone with my "Z"! LOL!