Sag2 mentioned the rear dif break in, but 500 miles is also for the transmission. The 68RFE has a "learn" mode, the 500 miles will allow the transmission to learn and then it will re-learn once you start towing a trailer. During towing when the transmission has re-learned the shifts will be shorter and harder, this helps to reduce wear on the transmission, the downside with this is when you unload your trailer it will shift hard and seem harsh, so the trans will then re-learn. So, long/short is the break in is more for components other than the engine.