Engine break in oils are balanced to protect the flat tappet cam, along with other high pressure contact areas, but also allow the rings to seat. What if you're only changing the camshaft, and you don't need the cylinder wall and ring wear?

You still need break in oil for the cam, it won’t affect anything else. Just coat the cam with assembly lube, roll over engine n make sure lifters rotate.

the last flat tappet cam i ran, I ignored all the std protocols as it was installed one day & racing the next. I used Royal Purple 10w40 syn oil, no additives. I ran it up for 20 mins to run the lifters in, reset the clearence, let it cool down, gave it another heat cycle later that night & the following day I went racing. That engine was abused for 3 years before refresh, the cam & lifters looked great, the bores were a little glazed, but for running synthetics from day 1, i thought it was in good condition. I was using a Comp Cams cam & lifters.

I used Joe Gibbs break-in oil and had good results with a fairly aggressive flat cam.

Gus

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=JEx2l9BF2Ek

Go 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