Yes, another thread about the break in period. HOWEVER, I hope to shed an educated light on the debate. I work as a reliability engineer. A what? An engineer concerned with prolonging equipment life for an industrial plant. I've had training/certification in lubrication analysis, vibration analysis, infrared, bearings maintenance, electric motors, etc.
Nissan recommends the following:
During the first 1,200 miles follow these recommendations to obtain maximum engine performance and ensure the future reliability and economy of your new vehicle.
Avoid driving for long periods at constant speed, either fast or slow.
Do not run the engine over 4,000 rpm.
Do not accelerate at full throttle in any gear.
Avoid quick starts.
Avoid hard braking as much as possible
I agree with Nissan's 'slow and steady' break in period. Here's why. When breaking in an engine, it is really not likely that internal machine tolerances are THAT out of wack; Nissan has probably gotten pretty good at machined parts and fitment. If there are any slightly off spec tolerances, you would want them to wear in gently and evenly. A gentle break in period allows for this. If you run the engine like you stole it, you risk inducing stress concentrations in machined parts and microscopic fatigue/cracking in the metal. This fatigue will NOT be noticeable for quite some time. But give it thousands of miles and a couple years, and those cracks propogate; fatigued metal begins to spall (tiny chunks break off). Other engine parts need to wear themselves in as well, such as bearings. Again, there should be no unreasonable machine tolerances in a new engine. But machining processes are not perfect, so tiny imperfections may exist, and a gentle break in period allows components to wear in in a 'safer' manner.
"If you don't run it hard, you won't seat your rings and you'll lose power"
Honestly, I do not know about specific engine parts, nor have I worked on them. But I find it hard to believe that one must run an engine hard to get the (piston?) rings to seat properly.
"Gentle break in promotes carbon buildup! It'll kill your engine"
I HIGHLY doubt that a new engine will have ANY carbon deposits after 1,200 miles. If it does, we're talking about MINUTE amounts that will dissipate from spirited driving after the break in period. Unless you're just using absolute crap for gasoline (and I mean ABSOLUTE crap). Not to mention carbon deposits occur over time and are largely influenced by the maintenance you perform on your engine over its life.
Just my two cents
Hope it's informative to some.