I recently saw something pointing out another wrinkle about taxis -- per mile, their engines go between hot and cold a whole lot less, so there's less stress from thermal expansion. Once they're on, they pretty much stay on for at least the driver's normal workday.
I don't know how much difference this actually makes, but common knowledge amongst the people I know seems to be that the most stressful time for a normal engine is starting up.
It's not just common knowledge, it's the truth. And it makes a huge difference.
When an engine starts up, all the oil is sitting in the bottom. In a good condition engine, the oil pump starts giving meaningful pressure the moment the starter turns, and starts pumping fresh oil throughout the engine.
However, when cold, an engine has the wrong tolerances to account for when the materials heat up and the materials expand. So the oil pressure isn't quite right.
As the engine ages this problem gets worse, so each startup cycle gets progressively worse. This is why a car with a worn engine will show the 'oil pressure' light for an increasingly long time after it's started.
The length of service life for an engine will come down to
a) operating hours (not just distance)
b) operator abuse (revving while cold, excessive RPMs throughout use)
c) service attention (oil changes, filter changes, coolant changes)
d) duty cycles (how many times it heats up and down).
The worst thing you can do for a car is a lot of short trips with a big enough spacing to let the engine cool, and aggressive driving while the engine is still cold.
An F1 engine is seized when cold, it requires several hours of warm water and oil to be pumped around to bring up the metals to the operating temperature.
Modern engines can go a very long way if cared for properly.
I don't know how much difference this actually makes, but common knowledge amongst the people I know seems to be that the most stressful time for a normal engine is starting up.