A question for the engineers here.

[shp4man]

In the recent past, like 30 years ago, a vehicle's engine was generally good for 150K miles, if well cared for. At that point, wear would take over, the engine would start using oil and need an overhaul.
Fast forward to now, and it's common to see engines with over 300K+ miles running around and never having  been rebuilt.

Why? Was it improvements in metallurgy? Machining? Tighter tolerances? Oil? Something else?

[MrH]

Yes. 

[MX793]

All of the above

[SVT666]

You answered your own question

[Soup DeVille]

In the recent past, like 30 years ago, a vehicle's engine was generally good for 150K miles, if well cared for. At that point, wear would take over, the engine would start using oil and need an overhaul.
Fast forward to now, and it's common to see engines with over 300K+ miles running around and never having  been rebuilt.

Why? Was it improvements in metallurgy? Machining? Tighter tolerances? Oil? Something else?

Better fuel and ignition management. All the other things you mentioned have been pretty constant (machining has actually gotten a little looser).

But, cars are no longer running rich at start up, leaning out as they warm up, or using valve float as a rev limiter.

[Eye of the Tiger]

Why do you need an engineer to answer this question? I'll answer it for 1/4 of the price.

[hotrodalex]

Why do you need an engineer to answer this question? I'll answer it for 1/4 of the price.

I'm not sure any of your previous cars would last for 150k after what you did to them.

[Eye of the Tiger]

I'm not sure any of your previous cars would last for 150k after what you did to them.

My cars are irrelevant. Vehicle manufacturers should hire me to perform destructive testing.

[hotrodalex]

My cars are irrelevant. Vehicle manufacturers should hire me to perform destructive testing.

That's an excellent idea.

[GoCougs]

IMO why engines last longer in general these days:

1.) Block design. Dimensional stability = longer lasting bearings/rings. Just look at the the two-bolt bottom end of the average small block V8 vs. the 4-6 bolt bottom + structural oil pan nature of a modern block.
2.) Head design, specifically OHC. Pooprod valvetrains put a lot of force (wear) on valve guides and cam lobes.
3.) Higher quality rings and bearings.
4.) Higher quality manufacturing tolerances and processes.
5.) Higher quality engine management (specifically, to prevent wacky A/F ratios; too rich = tend to wash the cylinder walls of oil + dilute the oil).
6.) Higher quality oil (or more specifically, oil nowadays is much more tolerant of too long change intervals).

IOW, because they're designed to last longer. A '75 Monte Carlo body/frame/interior wouldn't last much beyond 7 years and 125,000 miles so why design the engine and transmission to do the same?

[Speed_Racer]

Remember how cars used to only have 5 digits on their odometers? Crazy how much better they are now.