[Barry_R]

A few thoughts - in no particular order

15 pounds at idle is not going to hurt anything ever. As long as it goes up smoothly with RPM you're gonna be fine. That is not the problem.

10W-30 oil is not the problem.

Not enough oil in the pan is not likely to be the problem - not at idle for sure.

Pulling the valve cover is a very good idea - but you should see signs of plenty of oil dripping and draining back even without a running engine.

You want at least a full turn of lifter preload on a normal FE - this ain't a small block - the adjuster is direct on the pushrod instead of running against a fulcrum. Sounds like you're already there.

The original builder looks to have taken the effort to shim rockers for position. Looks fine to me, but I obviously cannot see that everywhere.

My bets - for what they are worth:
Debris in lifters
Worn rockers and/or shafts (they are 40 years old)
Broken or really weak shaft springs - I rarely use those anymore
Pushrod to intake contact - not likely with stock parts - but he did shim rockers over