I was asked this question recently... In my opinion, new engines, if they are to last a long time and be economical, must not be run in, in the normal way. The only type of driving that should be avoided in the first 1000kms is a constant speed, I.e., a long run where the engine spins at the same rpm for long periods. In my experience, an engine that works hard from day one is the engine that uses less fuel, lasts longer and uses far less oil. Engines that are molly-coddled when new always use oil and are under-powered and often do not deliver power equal to spec. This is my experience through several used and many new vehicles I have owned.
Typical examples are my three Land Cruisers 4,2D engines. My first two I ran in for the first 3000 kms, avoiding full throttle and even warming them up. Then my current one. From day one it towed a trailer, foot flat, maximum revs and punished. Today this engine uses no oil at all (166 000kms) and produces more power than Toyota spec. It also uses 10% less fuel that the other two, even though the vehicle is heavier. It is quieter and a much nicer to drive than the others. Other than the way it began its life, nothing makes it different from the other two.
Another example is all Lycombing aircraft engines – large block, long stroke petrol motors – have large red labels on the crate: ‘NB. DO NOT RUN THIS ENGINE IN. USE MAXIMUM POWER FOR FIRST 25 HOURS. It’s because if they are run in, the rings never produce a good seal and a microscopic step is created in each cylinder wall at the point where the piston is at its highest position. But when the engine revs higher, the conrods stretch, and the rings pass this ‘lump’. The result is lost compression and all that goes with it.
A last example is racing car engines. They are NEVER run in. Their builders know that it has to be flat out from the first moment they run, otherwise they never perform properly.
If anything running in means the opposite of what is common perception. Work the engine normally from day-one, and it will perform better all its life. I am not suggesting abuse it, but demand full power often, vary the revs and power demands, and get it to operating temperature as fast as possible. And the kiss of death for any new motor, and even an old one, is to warm it up while at idle.