I'm working on a '91 Ford Exploder with a 4.0L V6. Has 122K on the clock, and runs like a champ. But it also sucks down the oil like there's no tomorrow- as in a quart about every 150 miles.
The exhaust, oddly enough, does NOT spew large clouds of oil-smoke. In fact, the exhaust appears clean and normal. There's no immediately-apparent external leak either- there's the usual spots of slight ooze-level leaks on the outside of the engine, heavily-encrusted with old dirt just as you'd find on any 100-thou mile motor. No "fresh" stains, and other than the few spots, the rest of the undercarrige is clean and dry.
So where's it going? Is it burning it? Pumping it somewhere else? If it's burning it (bad rings, bad valve guides or seals, etc) then something that sucks up a quart in 150 miles should have a huge blue cloud following behind it, cat or no cat. This thing barely gives a wisp, if that. If it were leaking it overboard, the undercarrige or the bottom of the engine should be soaked, and it should be leaving puddles wherever it goes, but it doesn't.