I have rebuilt many racing engines and since they are balanced and built to very high tollerances, we broke them in with hot laps right out of the box. As for production line cars, I would take it easy for the first 3,000 to 5,000 miles making sure to alter your rpm's and use the different gears, also take it easy on the brakes as well. I believe the manufacturer uses a non-detergent break-in oil as the first fill.