I've heard lots of talk about "cam break-in" over the years... and I tend to think it's just that- talk. "Break-in" procedures that I've read about vary widely, but often involve some sort of magic assembly lube on the camshaft, and keeping the engine RPM well above idle for a certain amount of time after initial startup.
I've rebuilt more engines than I can count- many dozens of diesels, and merely a dozen or so gassers. Here's how I do it on an engine with flat tappets: First, I make sure all the tappets move and spin freely in their bores. You'd be surprised how often the bores and/or lifters require minor polishing, and I suspect improper rotation is responsible for quite a few lobe failures. Second, I coat the camshaft and lifters with plain ol' engine oil- no fancy lubes for me. Third, I start it and forget about it... and run/drive it however I want to. Once everything is back together- regardless of whether we're talking about an engine in a car, pickup, forlift, tractor, or semi... I run the pi$$ out of it. If it breaks or leaks, then it never would'a held anyway. Never have had a cam problem.
Here's an idea for the brave of heart... I rebuilt a German-build John Deere diesel engine in a nasty old 544 industrial loader a couple years back. The factory manual specifically said to coat the entire camshaft (lobes AND journals) with black moly grease. I thought it was wierd... but I was feeling adventurous and did just that. It worked fine.
