These lights take a guess based on the sort of driving you've been
doing, and how much of it -- they don't actually measure chemical
properties of the oil. But the manufacturers think enough of the guess
to stand behind it for warranty purposes, so I suppose it's alright.
It's certainly got to be a better guess than "three thousand for severe
service, five [or six or whatever] otherwise."
As I understand it, if the computer decides your usage has qualified as
severe, the light will come on somewhere not too far beyond the
proverbial three thou anyway. At the opposite extreme, if you've been
doing mostly easy driving under good conditions, getting things good
and warmed up but not really overly hot and so forth, it'll save you
some time and money by allowing an extended drain interval.
Anybody know what fleet managers think of these things nowadays? Do
they just yank the car into the garage at intervals regardless of how
it's been used, or wait for the user to squawk about the light, or
As for Dallas (or wherever), I wouldn't flatly state that that's either
better or worse from a car maintenance standpoint. It depends a lot
on whether you get tied up in rush-hour stop-and-go traffic or have a
free-flowing reverse commute or what.
I would certainly consider changing the air filter a bit more often
than strictly necessary, since city air is often pretty grim in ways
that are hard to see with the naked eye. This isn't as big a deal in
modern cars -- they actually measure how much air is flowing downstream
of the filter, and/or how much oxygen is in the exhaust, as part of the
engine control loop, and thus automatically compensate for less air
making it through the filter. (Pre-computer cars just took a guess
and as the filter got dirtier the mixture would get richer.) But
you're still losing performance if the thing is dirty.
For city drivign, mind your brake wear as well. And if your vehicle
calls for a transmisson-fluid change interval, consider whether you've
done a lot of stop-and-go driving there too.