I've used Driven break in oil and also used Valvoline VR-1 w/ Comp Cams break-in additive successfully. Both were 440 Mopars.
That's the only two flat tappet motors I've built in the last 10 years.

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