Actually, Taxis may not be a good test bed for oil change intervals, here's why;
Taxi's are started in the morning and run virutally all day long, maybe even 24 by 7 with different driver shift changes. An engine that doesn't go through a lot of warm up/shutdown/cool off/startup.......... cycles does not go through as many of these cycles in a given number of miles. One of the problems is moisture and acid build up, this is what the additives in the oil try to fight. The typical engine situation would produce much higher acid and moisture deposit amounts.
The worst possible scenario for an engine is someone who starts the car every morning and drives it a mile or so to the office. Starts it that evening and drives it a mile or so back home. That's why it's common to see recommendations of X miles or Y months whichever comes first.
The test you cite is a very interesting one, I'm sure it would be much more difficult to perform such a study on cars used in typical service. It would take years, the taxi test probably took months, or a few years worst case.
Thanks for the great information,
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in