Not sure about what happens if you change the filter too.
But my personal experience from last summer was that when I topped up my TDI with 1 litre of oil (after a low oil warning), that the service indicator didn't give a damn and kept aiming at about 50k km, like it did before.
Of course, that 1 litre is only about 1/3 of the total amount, but if people suggest that the computer checks the status of the oil by measuring transparency then I would reckon that even 1 litre would help in making it just a bit more clear.
So I guess it depends (and it also SHOULD depend) on more factors than just oil transparency.
Moreover, servicing entails more than just replacing the oil and filter, so it would be very backwards if an oil/filter change could (substantially) lengthen the interval.
So unless you care about your car a lot, I think that premature oil replacements are a bit of a waste of money. The oil should last the interval. And the garage will replace you 'fresh' oil anyway at servicing, so why pay for long life oil and only use it for a (too) short period?
Enough guessing. That's my 2 cents...
Edo
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(2003 Silver 1.4 Tdi Exclusive(SE), 95bhp/235Nm chiptuned by ABT, black interior, sport seats, heated mirrors, TomTom3 PDA Satnav, JVC KD-SH909RB cd-receiver, Infinity Kappa 63.1 + Vifa on-dash tweeters, 12" Subwoofer on 300w Amp)