all the cam/lifter interface should show is some polishing of the contact surfaces... and the more valve spring pressure there is with a flat tappet cam, the more important this all becomes. People used to do a lot of things that you just can't get away with these days with modern cam profiles, modern oils, and modern valve spring pressures. I used to put new (mild street hydraulic with near-stock valve spring) cams in with just the cam lube that came with the cam, but never worried about spinning the engine over before starting and did just that to prime the oiling system (dry filter). I never even used break in oil. Never had a problem. But I would never try that today.Tis is true as long as the cam/lifters were originally out of the same engine and each lifter was put back on the same cam lobe. Anything else, then a failure is likely. Almost new doesn't mean anything to a camshaft - any measurable wear is unacceptable.
But I'm also puzzled why some people are so afraid of flat tappet cams and insist on roller cams to prevent failure. I'm running a Crower solid flat tappet with a fairly radical lobe profile (in comparison to most flat tappet cams) and have springs with well over 300# of open pressure. I also have a windage tray that I think kind of negates the 20 minutes at 2000+ rpm breakin (but I still did it). My solution was Joe Gibbs Driven engine lube, Lucas break in oil (for first 3 hours), priming oil system with tool, and most importantly Howards EDM hole lifters to directly oil that cam lobe. Valve lash settings haven't changed since I installed it last Winter. A set of EDM hole lifters are far less expensive than roller lifters......