Last few of mine have been flat-tapped (hyd or mech). Haven't used a specific break-in oil, but have added a bottle of the GM EOS. (I bought the last 10 bottles of that in my area a few years back, as I'd heard the formula was being changed.)
Nothing crazy on valve spring pressures, though those engines were pretty stout - usually around .510-.540 lifts, 130# seat pressures & around 400# max - but zero problems with any of them. Didn't remove any valve springs (I always use double springs). Napa Gold filters, either the Gibbs/Driven "HotRod" oil or Lucas, 10W-30. Local engine guru who does a lot of circle-track stuff sends oil out for testing & says the Lucas stuff is very consistent quality & amounts of ZDDP & etc.
Initial break-in is usually around 30 minutes, varying RPM & watching everything like a nervous Mom, haha.
After that I like to pull the filter & cut it open. Generally change the oil within a few hours or 100-ish miles, cheap insurance.
You'll generally see some debris in the 1st filter, maybe including some particles off the valve springs.