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Generally speaking, 911 engines use oil through the exhaust guides. A half decent cat converter will help to mask that, but if you do the math, a quart of oil in 500 miles is not a whole lot per actual combustion stroke. Seriously, it is about one one millionth of a quart assuming 2000 rpms at 60 miles per hour. That's 120,000 revolutions per hour for approximately 8.4 hours, or 1,008,000 revolutions per quart. So no radical smoke.
However, it isn't the greatest for your valves, because the wear is enough to keep them from seating perfectly, so they rattle a little bit while seating. Even so, maybe the next time you add oil, you should add a quart of ATF instead, and just drive it around the block gently for a few minutes. Then change the oil to something like Valvoline VR1 20/50, or Rotella T6 20/50, and run the sucker. Nobody on this forum really recommends the T6, but it is a decent oil with the correct additives, and it has a lot of detergent, not that you probably need it. Run that for a while, then change over to VR1, and drive it like you stole it for 500 miles, just to see if your rings won't free up a bit. Not that this is likely, but it is possible.
Then do a compression and leak down test. BTW, do you know when the valves were last adjusted ? It won't change your consumption, but it is a good thing to know, and it could effect your leak down test.
I'm not a doctor, and I don't play one on TV, but I have found that simple things like changing your oil and running your car can affect oil consumption. Castrol, in spite of its reputation, is not a really great oil, and can gum up your engine. Get it out of there.
Doubtless others more qualified will come in and give you some decent advice, but a little detergent oil, and a few quick changes has helped more than a couple of engines in my past.
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