Normally car manuals say to let the car break in gently, not accelerating or breaking fast for first 1,000 km or driving below 80km/h. Is that really necessary in today's world? Is there any proof anywhere that this makes the engine run better in the long run?
Maybe, it really depends upon the type of engine you are talking about. If you do some searching around, the usual reason cited for the break-in period, as summarized by Wikipedia is that modern engines are built with higher tolerances than previous years which eliminates the need for the break-in period. However, on smaller engines such as ones used for snowmobiles or or motorcycles tend to recommend that break-in procedures be followed to allow for proper seating of the piston rings and/or cleaning out manufacturing contaminates.
Every mechanical engine, gasoline, diesel or other (such as LNG) is made up of a multitude of parts manufactured to accepted engineering "standards."
These standards spell out, down to the smallest detail, what engineering criteria the finished part will meet. Quite frequently however these individual parts come from different factories and various countries. But because each manufacturer subscribes to the same engineering standards, a wrist pin manufactured in Korea will mate to a piston manufactured in India for example.
So far, so good. There is, however, the rub called "Manufacturing tolerances." Which are accepted as a necessity in the above engineering standards. Thus, the commercially produced wrist pin made above may be allowed to have a circumference + or - variance of say .0005" to .0009", it will still function for its intended purpose and meet the manufacturers requirement for MTBF (Mean Time Between Failure). The cost to get that same part to be dead-on perfect, with absolutely minimal deviation from its design specification would drive the cost much higher than the intended market (you and me) could afford.
That's one reason why the engine in a Lamborghini, Aventador Roadster costs so much more than the powerhouse of a Chevrolet Corvette: Manufacturing tolerances (i.e. variations from the design) in the Lambo are almost non-existent.
All of which Which leads us to: Breaking in a new engine! Let's say you could only afford the Corvette and not the Lambo, but you want your prized beauty to last a long, long time... What do you do? You break the darn thing in properly of course! Here's the engineering reason: All those parts in your new engine manufactured to that above mentioned +or- tolerance need to mate to each other (think of it as dating before marriage). Every part, commercially machined has microscopic hi and low spots on its surface called "asperities." These asperities need to rub against each other under load in order to literally break off the high spots and bring the load bearing surfaces closer to the ultimate goal of 100% contact. The other element is called "wear hardening." Piston rings, cylinder walls and camshafts along with many crank shafts in most non-exotic, passenger vehicle engines are made from nodular cast iron. IF these cast iron parts are allowed to break in properly, they will develop an extremely hard, long lasting wear surface. It may only be .001" to .003" or so thick, but it will stand up to many years of use.
This all happens on a microscopic level of course. But I guarantee you that, as they came from the factory, the rings to cylinder wall contact and the cam lobe to lifter contact etc on your new Corvette were NOT optimal for (a) full performance and (b) engine longevity. Breaking it in carefully will, however accomplish this.
Recommended break in procedures vary by manufacturer, but essentially it boils down to a few basics: (1) NEVER rev a cold engine! (2) Vary your speeds for the first 500 miles and don't maintain a constant speed (like 60 MPH) for extended periods. (3) In the first 500 miles, accelerate evenly and allow your engine to coast frequently (i.e. coast from 60 to 50 foot off he gas). This creates a high vacuum in the combustion chamber and pulls more fresh oil to the rings ad cylinder walls. (4) Change your oil and filter after the first 100 miles and again at 500 miles. (5) Only use high quality oil, preferably synthetic. I've rebuilt many types of engines and some of them exceeded 150,000 miles on that rebuild. Break an engine in right, treat it good and it will last you a long time.