I also feel the oil is a factor.
I have been using Mobil 1, 15W-50 since that is the only Mobil 1 viscosity that is designed for performance use and "muscle car era" engines. My last two cam break-ins have been using Rotella diesel oil with STP, then switch to the 15W-50. And no inner-springs... Other oils may be good to, but some lack significant amounts of extreme pressure, anti-scuffing, zinc, additive.

idling is bad for a flat tappet with heavy springs too.

1993 Daytona, 5.50 at 130mph (1/8) 1.19 sixty ft (PG). Link to 572 B1 - Part 1