Oil-Change: 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.