Taxi style use generally qualifies for shorter oil change intervals. Including that in the lease probably helps the vehicles stay maintained and resellable.
A number of people (particularly in the US, but not only) change their oil every 3-5,000 miles for reasons like:
- my father always changed oil every 3,000 (or 4,000 or 5,000) miles
- a friend of mine who races changes it every 2,500 miles so changing it every double that is appropriate
- the dealer (or the mechanic just around the corner) told me to change the oil every 5,000 miles or the engine will explode
Of course the Toyota engineers (mind you the same ones that designed, produced and tested for a few zillions miles the thingy that you chose to buy, implying that you trust them overall) saying 10,000 miles are a bunch of incompetent morons, what do they know?
http://toyota.custhelp.com/app/answers/detail/a_id/7604/rela...
Seriously, the 10,000 miles for (partially) syntethic oil is on the very low of the expected range of duration of a modern oil on a modern engine, i.e. it is a conservative enough duration, 12,000 to 15,000 is common.
And IF there is something that makes an oil or engine go bad is NOT "continued use" (like taxi use) but rather cold starts after long periods of inactivity (or racing, for other reasons).
In this the other also mostly psychological "limit" of "oil must be changed every six months no matter how many miles" has some more background reasons.
Still a (partially) synthetic oil is good for much more than that, usually 1 year at the very least.
Fully synthetic oils (very expensive) can last much more (but you need to change filters ever 10-15,000 miles anyway).
It's complicated, but basically an oil has two main functions:
1) lubricate
2) circulate and take away all impurities and metal debris (which are then - as much as possible - filtered and absorbed by the filter and - at least the steel ones - by one or more magnets), i.e. to "clean" (continuously)
So the oil has many components inside, some of which with different weight (i.e. that may tend to separate) some of them that may be affected by water (the air that inevitably enters the engine can condensate) some by the oxigen in the air itself (i.e. oxidate).
Moreover some metal parts in the engine that are normally covered by a thin film of oil may well, after some longer period of inactivity "dry up" and thus develop some oxidation themselves or "run dry" thus creating more debris (that will be washed at next start and thus contaminate the oil).
And then there is the temperature cycles, an oil needs to be fluid enough when very cold yet do not become too fluid when heated, which adds the need of more components that may suffer from these cycles.
As an example in some industrial setups (think of static diesel electric generators) the oil in the engine is pre-heated electrically to keep it always from going under a given temperature.
All in all keeping it continuously mixed/moved and within a given temperature range is "better" for the oil.
"Extensive idling and/or low speed driving for a long distance such as police, taxi or door-to-door delivery use: Replace engine oil and oil filter"
[1] https://www.toyota.com/t3Portal/document/omms-s/T-MMS-17Coro...
Of course I take back what said before, always do what the Toyota engineers recommend.
Some engines are known to be more finicky to dirt (VW infamous chain tensioner comes to mind), and thus it makes sense, if you plan to keep the engine after the warranty expires, to reduce the oil change interval, as oil tends to be much cheaper than an engine rebuild.
There's a service where you'd send your spent oil to be analyzed for cleanliness, so you could determine the best change interval for your engine and usage. But that cost of that service wasn't much cheaper than just changing the oil a bit more often…
There's an even bigger reason it worked like that for so long. Car companies (American car companies) would print in their US manuals that you should change the oil every 3-5,000 miles, while printing in their European manuals - for the exact same car - that the oil change should be every 10,000 miles. I believe this was brought up on Car Talk once, for example. And they even went into a discussion of how it wasn't a units conversion problem - that the kilometers listed in the EU manual were equal to 10,000 miles, not that someone had just switched the units accidentally, etc.