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I burn about 1qt every 1k miles so in essence i am essentially adding new oil.
Quite a few years ago I listened to a petrolium engeneer extolling the virtues of synthetics. One of the things they did test was removing one quart and adding a new quart after a specified interval(s) and found that there were no advantages adding new oil to old. In effect, if this practice were followed, it would be a waste of money.