as long as there's no air bubbles in the oil coming up from the pump, the engine won't be able to tell the difference. At that point the main risk is just that the 2 quarts of oil are doing the work of 6, so it could break down faster. Considering we change the oil most of the time with lots of life left in it before the engine begins to be compromised, there's margin there too. It could be just fine, especially if they aren't driving crazy.