Our 1995 850 turbo sedan with 180k miles had an oil change about 2300 miles ago. It has never burned any engine oil during the last 15+ years in our hands. Early last week, when I checked the oil dipstick one morning (before I drove it that day), the oil level was at about half the distance between the full and empty indications; that is, in about ... 3 days, since the last time I had checked it before that day, about ~1/2 quart of oil was apparently gone! The following morning, I checked it again; this time, the oil level was back at the full indication again, although no oil was added at all. During the last 7 days, I have been checking the oil dipstick every morning, i.e., before I drive it at all. Some days it is at the full indication, and other days it is at about half the distance between the full and empty indications on the dipstick. Do you have any ideas of what may be happening? Is this a warning sign for anything that may be coming up soon? The car runs fine in every other respect.

1995 850 turbo sedan, a really great car
2006 S60 2.5T, a really nice car my other half likes very much