I've used Joe Gibbs Driven break-in oil in the most recent flat tappet motors I've built and haven't had any issues. No need for any additive. About 4 years ago I used Valvoline VR1 race oil w/ Comp Cams break-in additive on a 440 hyd. flat tappet build. No problems w/ that one either. These were all fairly mild hydraulic flat tappet cam street engines. Anything aggressive, I prefer a roller. 
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax
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