Your results look pretty good.
But, there are so many variables involved such as driving style, regional fuel quality/formulations, driving conditions (freeway, stop&go, dusty, daily/occasionally driven etc.) and such that I personnally don't consider this a rousing endorsement of a 15k/2 yr. oci, especially if you're storing the car seasonally.
Compared to the risk, an annual oci or <10k mi. still seems a smart way to go.
But, everyone is different, and it's their car and their scheckels, so let your conscience be your guide!
