<<Who's right? >>

Umm, you're both wrong. While changing your oil every 3000 miles will not hurt anything you are likely wasting your time and money. If you look at the maintenance schedule for the car it is very aggressive concerning fluid changes. This includes everything from brake, differential, coolant, etc. I really doubt they would have you change your oil less often than necessary. I think they recommend 6000-7000 miles depending on conditions and habits. I've changed mine about every 6000 miles on all my Subarus and have never had a problem. And I don't drive kindly.

Your friend on the other hand is setting himself up for problems. While Mobile 1 will stay slippery for up to 12000 miles it will be so full of acid build up that he will likely start seeing seal and gasket leaks before long. These acids will make the seals hard and brittle. If a main seal fails oil pressure is compromised and it doesn't matter how slippery it is if it doesn't make it to the area needing lubrication. Not to mention the mess on the car and driveway from all that expensive oil draining on it.

A good filter and new oil every 6000 miles and you should be okay. But the oil barons will take your money if you give it to them.