On a recent highway trip I experienced about 1 qt of oil consumed per 750-800 miles. This seems quite high. On a similar trip about a year ago I used about 1 qt per 1300 miles. Both trips were about 4000 miles and I measured/refilled with car cold and level, etc. so the figures are probably accurate. The car was loaded with 4 people and gear. The oil usage in stop and go city driving seems lower but we didn't measure it.

The car is a 95 850 NA and has had oil changes at about 5K intervals with 10W40 synthetic blend oils and decent quality filters. It has 130K miles, much of it city/suburban driving. There are no visible leaks anywhere on/under the engine including under the timing cover. The engine performs normally as far as I can tell with no diagnostic codes. It passed a recent emission test and is getting about 25 MPG on the highway (with the 4 people and gear). The rebreathing lines and flame trap have been kept clean and there is no evidence of blowby through the dipstick, etc.

We could not detect smoke in the exhaust at cold start, nor is any visible when pushing the engine hard, nor does the exhaust smell particularly of oil. The plugs look tan/grey and dry, not oily. The cooling system is clean and temperature gauge holds very steady.

The only observation that seems a bit odd is that the oil always seems to be very dirty (black) when it is changed. There have been a few air/vacuum leaks over the years (possibly allowing some unfiltered air in) but these we repaired promptly.

This car is otherwise high-functioning though not great looking and I've been intending to keep it.

So where is the oil going? I think it is being burned, but I would have expected some symptoms. Has anyone been using this much oil and what did you do?