@Bing ; If you have a new car, change oil at the interval the manufacturer wants you to. Otherwise you may void the warranty. Also use the eaxact spec of oil the manual calls for.
In case of the Mobil 1 Extended, that 8000 mile drain interval would apply only if your driving has sufficient highway miles to drive off sludge and condensation. If you live in Minnesota, park outside, do not have an engine block heater, and only drive 8000 miles a year, even the Mobil 1 Extended will not prevent your engine from sludging up.
Driving conditions vary greatly in the US and a normal mix of driving city and highway in the warmer parts of the country will allow 8000 miles interval with these premium oils.
As many others have said, we want to maximize ENGINE LIFE, rather than oil life and save a few dollars. Those posters who use department store oil and change frequently are also doing the right thing. In all cases meet the spec of oil the manual calls for.