"New" engine break-in period

Hi All, I have just completed rebuilding a motor for my truck. I've driven it about 30 miles so far, pretty gently.

Back in the day when you bought a new car, there was a "break-in" period for the engine, that was often quoted as 500 miles. You were supposed to be gentle on the engine and not really give it the beans during this time period.

So why was this time period necessary, and how much of that applies today? Reasons I can think of why it might have been necessary back then:

1. Some aspect of the engine architecture itself
2. Inferior lubrication qualities of the engine oil
3. Inferior metallurgy

Probably some other stuff I'm not thinking of.

If it's #1, then I would assume the same 500 mile break-in period would still apply. But I'm having trouble envisioning how engine architecture would have much of a bearing on this. Flat tappet cams, for sure, but my motor is the later hydraulic roller variety, so is there some other aspect of engine architecture that would require break-in?

If it's #2, then does an engine even need a break-in period today?

If it's #3, then what particular parts are we talking about? The engine has new rings, pistons, bearings and oil pump, but the same crank, cam, block, etc.

I understand that, in general, any new machine is going to have to "wear in" for a while, during which time friction might be higher. But realistically, how long do I need to baby it?

Opinions sought.

I think there are a lot of opinions on this, some supposedly have video proof.

My opinion is to do what the manufacturer suggests. Might be harder to find that info now though.

In the years I worked on aircraft that’s always how I (we) did it.

The first 20 minutes are critical as the cam and lifters need to mate to each other.  And the guidance I've seen says to hold ~2000 RPM for those 20 minutes.

After that there's a period of time during which the bearings need to bed and the rings need to seat.  The rings need some pressure on them in order to do that.  So that's why they say that there needs to be acceleration to put pressure on them, but then backing off to allow things to cool.

There is no definitive "period of time", but that's where the 500 mile figure comes from.

I'd say you definitely need some sporadic pressure in the cylinders to force the rings to seat against the honed bores.
It's really no good if the cylinders get glazed and you end up with an engine that burns a lot of oil.

Oil additive packages have definitely changed since the '80's but I don't know of any machine shops that are using much finer hones on older tech iron blocks.

You should probably ask your machine shop what kind of rings went into it and what kind of finish they put on the bores