Go with a break in oil formulated for flat tappet cams. After break in, go with a specially formulated oil for flat tappet cams as well. Additives may work 99 percent of the time. But there can be conflicts when mixing additives and oils, and a few are not compatible. I avoid it when I can.
The main thing is to make sure those lifters rotate when you turn the motor over before firing. Did your engine builder check? If they don't rotate a little, that MUST be corrected. They don't need to move much at all, but they have to turn to live. Mark all lifters a 12 oclock with a felt tip, and turn the motor over a couple of revolutions. It is simple to fix one that doesn't rotate, just remove the lifter that is not rotating, put some 500 grit sand paper in the palm of your hand and rotate the base of the lifter against it till the shine goes away. Clean, and reinstall. Recheck, and fire the motor.