The guy that does my machine work uses Valvoline 15w40 to break in any flat tappet engines he builds. Hasn't lost one yet. Attention to detail ie make sure the lifters all rotate is at least as important, if not more so than the oil I think. Nothing leaves his shop unless it's dyno'd first. No dyno, no warranty.
https://www.youtube.com/watch?v=JEx2l9BF2EkGo to the 6 minute mark to watch the Pennzoil H300 win the HRL Hydro 350 Final with his engine in Cambridge MD this past weekend. They run flat tappets WFO at 7200+ for upwards of 5 minutes with warm up laps. Course is 1 mile, 5 laps IIRC.
He had 2 engines in the Grand Prix class for a team from New Zealand, one came in 4th in the Final and the other did 2nd in the Consolation. Miscue on the tune up or the outcome might have been much better. 468 blown injected Methanol, 1300-1400 HP. No flat tappets there tho...
Kevin