Well, I would put money on a clogged passage way first or an oil pump problem second, if the engine has set awhile the "O" ring for the oil pump might have cracked causing a siphoning and no oil getting up into the engine after pressure has built up, that was a huge problem for the cadillac engines in the 80's,For clogged oil passageways I would try putting in about 2 quarts of marvolous mystery oil or kroll oil with fresh oil and a new filter and run it for a bit that way, let it run at temp for awhile to get things hot (say 20/30 minuets?),let it set for a day or so, repeat about 1/2 a dozen times then go back in and see what it dose then with oil flow and check the oil for black deposits, if it shows black flecks in the oil keep doing it another 1/2 dozen times or more, I used to do that with babbet beater stovebolts to free them up. The only way the gaskets could be in the way is if they are poor grade repos or too much silicone if they had been rebuilt, dose the engine look like the heads have been redone? or are the heads black and tar looking under the valve covers? Another problem is a cheap or dirty/clogged oil filter that can't keep up with oil flow and causes a sort of vaper lock, The only other thing I can think of is a worn oil pump shaft but I have only see that once.
