My understanding of it is there are two issues

1) The quality of the oil, and how long it will keep providing adequate lubrication
2) The accumulation of contaminants through the combusion process, which occurs at the same rate regardless of the oil used

So the need to change oil is a mixture of both of these two, as below

$10 supermarket oil..... 5k changes (because this oil is poor quality and will break down quick, and before contaminants accumulate excessively)
$30 magnatec ..... 10-15k (better oil, contaminants accumulate and need changing at this distance)
$70 synthetic ....... 15k+ but don't overdo it! (better oil again, but contaminants still accumulate, so still need to change at 15k, mainly due to contaminants, not oil quality)

Service intervals have been extended in the past by manufacturers simply by increasing the sump capacity eg) 4 litres to 5 litres. This takes longer then for the contaminants to accumulate to the same concentration.

Really dirty running engines, like the landcruiser 1HZ diesel, have oil changes every 5k. Not due to issue1, but all about issue2.

So synthetic oil is great & lasts a long time (issue1), but the contaminants will still buildup in higher & higher concentrations, unless you change it regularly (issue2).

Ahh yes and of course there is the temperature issue.
If you do lots of short trips then the oil get contaminated quickly.
long runs where the oil gets up to temperature and the whole engine settles down nice and warm it burns off the crap to some extent and the engine lasts longer. IE country kays = high milage but a long life motor.
Ifd you go to the shops and the station and round the local haunts constantly like I have been doing then shorten change intervals. If you ran the car constantly for a week like a taxi then you may hardly get any contamination. This is probably why they can do 1000000klm without the sump off.

richard, yep you're spot on
but the effect you're describing is really better called dilution in my view, not contamination (but that might just be a personal thing & it depends what you were meaning)

the stop start driving you describe is called "severe operating conditions" in the owners manuals, and it's bad news

even a taxi will still get contamination however, that's just a fact of life of burning fuel
lots less contamination in gas, but still there (acidifies the oil)
and lots when you run petrol

even country car gets black oil when they run on unleaded (ie. contamination)

so it's normal oil, every 10k for me on a straight gas car

i've been told that penrite HPR 10 Gas is the best to use but i guess every one has there own opinion. Maybe try it out and see what you think.

too many people talking out there a** in this thread

just change your oil semi-regularly, spend what u want...get over it, its just a 10y.o. ford!!!

its not a fricken ferarri