very good question!  to skip forward a bit, there's quite a bit of
movement in the industry towards pushing out oil change intervals.
there's two main methods - one is using the ecu to log usage and
electronically compare that with a table of oil conditions. the other is
 measuring dielectric constant and mapping that in much the same way.
the expensive option is actual spark spectroscopy analysis like they use
in f1, but i'm not aware of that being used in production vehicles.

biggest reason for hour-based changes vs mileage is that with a lot of
diesels, [generators, etc.] there /are/ no miles to log!  farm gear
doesn't cover a lot of mileage either.

regarding discrepancy, big commercial diesels tend to have a much more
regular life than the average passenger car.  not only do diesels wear
less on cold start [diesel wetting a cylinder wall is a lubricant, not a
dilutant like gasoline], they tend to have a lot fewer cold starts too.
 a lot of trucks run for days on end, marine diesels for months on end.
 this means the oil has less crud in it from the cold start process
which helps.  then there's filtration.  a lot of big diesels have two
filters - one is full flow, the other is bypass.  the bypass filter
pulls out all the fine stuff the full flow filter doesn't, and this
keeps the oil in pretty good shape.  a lot of commercial fleets are also
supported by oil analysis and usage logging.  a vehicle used on a
regular route can have its oil change strategy "tuned" very accurately.
 hardly any passenger cars do this, so manufacturers are really trying
to cover hugely different usage patters with their recommended service
intervals.

but the bottom line on all this though is what the market supports.
even my old 89 civic has a 7500 mile change interval.  yet who here
waits that long?  the later civics have 10k as their interval - at
30mph, that's 333 hours, not far off john deere's diesel spec.  again,
there's a lot of resistance among american consumers about extended
change intervals with people routinely ignoring the onboard diagnostics
and getting oil changed 2 or even 3 times before the oil service
notification comes on.  i don't see us embracing much beyond 10k unless
there's a massive public education campaign, but that's against the
interests of the oil industry, so that ain't going to happen!  otoh, i
hear that the europeans are very keen on long change intervals and for
pushing it out as far as possible with up to 30k on some of the euro
cars.  of course, improved lubricants play a significant role in that too.

My guess is that diesel engines need service more by hours of operation than
gasoline engines do. Couple that with the sort of duty many of those engines
get - miles traveled not being a good measure of usage and sometimes being
used in stationary applications - and doing those by miles is pretty
far-fetched. Airplanes powered by gasoline engines are the same way.
Maintenance is done by the hour meter and probably still would even if there
were an odometer to read.

Disclaimer - I am not very familiar with diesels. I've been driving a Ford
F350 diesel at work for about a year and have operated a diesel snow cat.
Interestingly, although the snow cat has an odometer, its fuel consumption
is estimated in terms of hours of operation rather than miles.

Mike

Oh, yeah... an OEM thermostat!