Thanks very much for the helpful advice! Here’s the information I have:
The engine has 102K miles on it, and if my records are correct then it burns almost three quarts per gas tank, and each tank goes about 225 miles. (This is significant, because I’m paying $18 for a tank of gas and almost $10 in oil!)
I can confirm that it does not actually LEAK any oil, it’s stored in a clean garage and has not marked its territory. When I start it up there is often a small cloud of blue smoke, but this is the only time I see any sign of oil burning whatsoever.
Even more frustrating, I changed the oil immediately after buying it. I am not due for a change for another 1,000 miles - and yet the dipstick is sludgy and black.
(Edited for grammar)