With a car the age/mileage of yours, you are checking your fluid levels regularly, aren't you.

I suggest AT LEAST every two weeks, weekly is better.

If you check with Volvo, you will probably find that using a quart of oil every 1500 miles is within spec, however you should not let it get more than 1/4 of a quart low.

The limit for oil burning would be causing a "check engine" light or fouling the car's plugs.

You should check the flame trap as one 850 we have stopped using oil when we changed the flame trap. I had been told that it was clean a couple of times since 98,000 miles, six years ago,

When we changed it at 180,000 miles, oil burning on highway trips stopped!
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 435,000 miles put on 9 bricks