Thought I would share my recent oil analysis results. I've switched everything I own to Amsoil but I've never really gone crazy with the extended drain intervals. They are saying 15,000 miles with a turbocharged vehicles and the EaO filter. I'm about to try it.
My '02 was switched to Amsoil 5W-30 shortly at about 30k. I changed the oil and tested it at 37k and 54k (changed once around 46k, no test). The results keep coming back good. I add half a quart about every 5,000 miles. This last test reflects about 3300 miles between tests - 2,300 miles of that driving from NY to CO with a 1000 lb snowmobile / trailer in tow + a car full of stuff (guessing about 2000 lbs). South Dakota was 106 degrees with a headwind and the car vapor-locked on I90 (at least a vapor-lock is the best explanation). Then I dragged it over a mountain pass in CO to finish it off. Total mileage on the oil about 11,300. I expected them to recommend changing the oil - and the test came back good again - with a recommendation to run the oil another service interval. I was blown away. I changed the oil anyway - but this time I'm planning on going the full 15k.