Admit it, it is something we've wondered at one time or another, right? In my teens I had a friend who owned a Rambler. It burned about a quart of oil every 500 to 1000 miles. He decided there was no point in changing it since it was effectively changing itself every 5000 miles. In fact, we thought the car was better off because it always had 2 quarts in it that were less than 2000 miles old.
It ran like that for several years until some f&^kwit stole the thing and ruined our experiment. If memory serves, one of the Car Talk guys, Tommy, I think, did the same thing to a Dodge Dart. I don't recall how the car died, but it ran for a long time.
The key is that the consumption/replenish rate be high enough that the chemistry of the fluid is constantly being renewed.
