As an aside, my dealer uses Castrol part-synthetic for oil changes. The
mechanic told me (unofficially, of course) that it's safe for at least
10k miles as long as the filter is changed each time.

It all depends on how the car was driven ? Is that city miles or
highway? or a combination.

If its city miles with few highway miles , its not the colour of the
oil that will determine the oil change interval , but the accumulation
of acids and water in the oil.

A car driven mostly on the highway at highway speeds , (above 60 MPH,)
will burn off all water and acid in about 20 miles of this type of
driving , because the engine truly gets up to optimum operating
temperature.

In  city driving the engine oil and engine seldom if ever reach the
same temps as when driven for long periods on the highways.

If you drive mostly in the city it would be adviseable to change the
oil& filter  every 5000 miles

If your drivng is mostly highway you could probably go at least 7-8000
miles without a problem with full synthetic.

I change my oil every 5000 miles and I drive a 50-50 combo . of city
highway driving.(spring and fall)

After all, oil changes are cheap maintenence,and I use Mobil 1
synthetic 5w30 in the summer gray cap and yellow cap in the winter
(extended service)

Hope this helps?
Ed

Hi,
Today's oil is far advanced compared to years passed by. synth oil
every 6000km? You must have lots of money to waste. Is your car turblo
charged one? Even regular 5W-30 will last 10k Km in normal driving. Do
you replace spark plugs every 6 months too?, LOL!