Culled from online sources...
Breaking in a new vehicle is really about the engine. The break-in — or mechanical run-in — period is designed to begin to wear the engine evenly and smoothly with low, consistent pressure, normal operating temperature and smoothly flowing oil. The goal is to get the engine's piston rings, which expand, contract and flex, to seat properly on the cylinder walls. If there are imperfections in the pistons or the cylinder walls from the manufacturing process, working the engine too hard and too soon can wear down those imperfections too quickly. That leads to "hot spots" within the engine's cylinders, which can cause problems in the years to come.
Proper, manufacturer-recommended break-in procedure is designed to enable the engine to do what it needs to do. The benefits, are better fuel economy, better performance, less chance of burning or leaking oil and overall longer engine life. For instance, Nissan suggests its GT-R should not be driven at more than 50 percent throttle or over 3,500 RPM (revolutions per minute) for the first 300 miles (482 kilometers).
How do I break-in a new car engine?
Thankfully, the break-in period for a new vehicle doesn’t last very long. An engine should be ready to in 500 to 1,000 miles. If an owner has a long commute, this probably won’t take very long. During that initial period, experts suggest you adhere to the following suggestions:
• Stay out of the red — When someone says ‘redlining the engine’ they are talking about pushing the engine to its maximum RPM. If your vehicle has a tachometer to measure RPM, top-end of the scale is usually denoted by red numbers or an actual red line. For the break-in period, avoid mashing the gas pedal which will push the engine harder than necessary.
• Shift on time — A vehicle with a manual transmission will provide the driver with some extra control. Experts advise keeping the engine out of the red by shifting gears before the tachometer reaches that point.
• Avoid cruise control — Cruise control is one of the most popular features to become fairly standard across the industry. An engine needs to work at different RPM settings to be properly broken in. Cruise control sets the engine at a constant speed which isn’t great during this time period.
And from this Forum: Rogue Break-In Period?