This is not really scientific, but I did oil changes on our Mazda 6 at 2.5K for the FF, 5K OCI for the next one, 6K OCI for the one after that, and am now on a 7.5K OCI. Like I said, not scientific and no UOAs to back it up, but it works for me and I am sure that the car is being treated well. After the powertrain warranty expires, I plan to go to 1 year or 10K miles (whichever comes first).