If the honeycomb inside the cat gets coated with oil it will fail and the CEL will come on denoting this. It does not take much oil to destroy a cat. The basic workig are inside the cat is the platinum/palldium/Rhodium substrate that is the actual catalyst. The substrate can burn off some oil, but large amounts can not be burned off, and the substrate will stop doing their magic. So I doubt if a lot of raw oil is passing through the combustion chamber in into the cat. I'm guessing the engine is burning it rather effectively and all engines do burn some oil.
In those mountain areas, on highway driving, etc., you will probably be pushing more oil through the ventilation system and burning it. Remember, every time you go into wide open throttle your engine becomes an air pump and pushes oil through the system, under load, half-throttle going up hill for example, you will also push some through the system.
I use about a half quart of oil for each oil change at around 5-6K. Checked it this weekend, with a little over 3K and it might have been down by a quarter of a quart; really hard to tell. But my H3 does no highway driving, only a daily driver and on weekends taken out on some easy trails (by myself) or once in a while on some decent trails (with Jeep / Nissan people). But last year, in November, when I towed my trailer down I70 to Zion NP and back, I used about a quart and a half of oil, but there was a lot of up and down some decent long grades at highway speeds (i.e., wide open throttle).
Funk could also be correct, in some cases it might be worn guides, seals, etc. As these engines age, interior parts start to wear. It all depends on the engine and how it is driven.
