Mfgs do the best they can, but there will always be the possibility of small metal shavings, or parts getting 'shaved' during the break in process. When you have new metal rotating against each other (think bushings, thrust washers, regular washers, etc.), they have to settle in and get seated. If you don't change your oil on a new engine, those shavings will scar the engine. Just do like the Mfg says. Very cheap compared to a new engine.