Change oil of your car: How often?

Nowadays most of the automakers specify 5000 or 7500 miles'
interval for oil change. But most of the shops in the business of car
repair still recommend 3000 miles' interval.

It depends on your driving habit if we wish to decide who is right.
Even if your auto-manufacturer recommends longer interval, it could
be that they are actually meaning it for a driving habit that does not
involve wondering around the city with too frequent starts and stops
or carrying excessive loads. Short (like 1-2 miles) intermittent trips
in too cold weather and long (above 25 miles) continuous trips in too
hot weather (say in California desert) or any trip in muddy roads
may also go against such recommendations.

On the other hand, if your driving habit is quite normal most of the
days, it is good to have oil-changing trips at longer intervals. This
saves your money and wait-time and also prevents some significant
amount of pollution to the environment.