If it's never been apart in 260K miles then it's rings, valve guide seals and most likely the intake manifold base plate gasket. Look down through the throttle plates with a strong beam of light and if you see a pool of oil, there is your answer to that amount of oil lose. Should be fouling your plugs with that kind of oil usage if you are burning it, unless you are working it real hard.
That's pretty light oil (10w-30) for that number of miles. Surprised your oil pressure isn't low. You can try some 15w-40 but it's not a fix. Oil is cheap vs a tear down/rebuild if it's not a keeper!
