On a more simplistic train of thought... You've got 275K on it, and it doesn't leak or burn oil. Is there really a compelling reason to change what you've been doing?
I run Mobil 1, but not because I'm some oil snob. I just want to simplify, do I run the same in my cruiser, my wife's cruiser, the Avalon and the kid's Toyota's. I change it whenever... No oil related issues.
Similar reason here... i run M1 EP on both our cars. Its easy to change oil once a year on two cars.