The test never concluded that extended change intervals were not harmful to the engine. They merely concluded that Blackstone's test said that the oil was still ok to use. Only a complete engine tear down will reveal the difference between normal and extended change intervals. I am unsure what metric Blackstone uses, but it is possible that even if their measurement says the oil is still good, it might not be anymore.
Like I posted in the oil filter thread, I think that synthetics can go longer, but it is so easy to change the oil, why not do it more often than not? Frequent oil changes mean no question of engine condition, and no costs needed to do oil analyses.