There's no way you can predict how long it will take before detergent packages will deteriorate or how long it will take for oil to shear
for all engines and environments.
I do 10K mile oil changes on dino oil - but, I spend $25 extra for oil analysis to make sure the oil that came out was still effectively serving it's purpose. If it wasn't, I'd shorten the oil change interval for next time. For me, that has not been the case
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Here's a Mobile 1 super syn experiment with oil analysis every 1K miles - it lasted 18,000 miles
http://neptune.spacebears.com/cars/stories/mobil1.html
This one with Amsoil - 12K miles
http://neptune.spacebears.com/cars/stories/amsoil.html
Both tests show that adding oil as needed will extend the life of your oil by a great deal
Keep in mind that in the above tests -- the oil filter change is not the reason why metals dropped and rather, it was the additional oil added that lowered the amount of metal (the oil analysis looks at metal particulate smaller than the filter can handle).
But again, ymmv
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Time is the best teacher. Unfortunately it kills all its students.
Bike Miles (Begin Aug. 20 - '07): ~433.2 miles
11/12