what oil to use
I would suggest valvoline RACING OIl, 20-50. your dealer might have to order it but is good stuff and not real expensive. make sure you specify racing oil or you will get the regular valvoline. this race oil also has the proper amount of zinc in it which is a big problem with oils today (no zinc) which ios a big problem when breaking in flat tappet cams( they wear out lobes on the break in. good luck. PS you may stiil want to see about a better oil pump or check the other things that cause such a big pressure drop like yours