You will spend hours reading opinions on this. Most new engines are bench tested from the factory so the initial 'break' in of rings is mostly done given the tighter tolerances of modern manufacturing.
The Nissan recommendation is conservative. Personally I feel the best part of it (and consistent with other manufacturers) is to avoid steady state operations, i.e. cruising at 3K for an hour.
Vary the load on the engine, which implies varying the RPM load. You don't have to bury the throttle to get it above 4K, but IMHO its good to stretch the legs of the RPM range once warmed up. Sure you can get plenty of driving done below 4K but pushing to 4500, 5000, 5500 as you progress in the progress through the break in period is a good thing.
I have only broken in two new cars myself. First was a high performance honda which I did take on the Highway a bit when young. Next was a 2002 WRX that I very much broke in by varying load for 500 miles or so. Honda was sold at 140K, the WRX has 125K and still going. Both barely used any oil between 7500 synthetic changes.
Rest of my cars have been pre-loved so I have absolutely no clue how they were used. 2010 Z and the 06 Cayman.
We drive both of them with gusto.
- b