oil consumption

I have a 1987 MB 190E with 56,000 miles on it. It consumes a large amount of oil. The motor doesn't leak a drop of oil, and I replaced the valve stem seals about a month ago. I took it on a 220 mile trip a week ago and it consumed about 1 quart of oil. It doesn't smoke and the compression tests 125 lbs. per cylinder. The motor is very quiet. I have been told it's the valve guides, but other people told me you can't pour that amount of oil down the guides in that many miles. Is there a way to tell definitely what the problem is? I have had some places tell me that they had real low tension oil rings in them and if the oil wasn't changed methodically they got stuck and that was the problem. I would appreciate any help you might be able to give me to solve the oil loss.