I spent some time at the library researching lubrication and engine wear in internal combustion engines. A great reference is the volume on tribology and wear of the Metal's Handbook for those interested.

For best engine break-in, you should run a richer oil ratio and light engine loads. Just like the engine manufacturers recommend. You should not run the engine at full throttle for extensive time during the break-in.

The reasons are as follows:

There are 3 regimes of lubrication.
1. hydrodynamic
2. mixed
3. boundary

If you have a complete oil film separating the parts, then there is no metal contact and zero wear. This is hydrodynamic lubrication - an example is a journal bearing with high pressure oil between the parts. The hydrodynamic regime is favored by light loads and high speeds.

If there is an very thin or incomplete oil film separating the parts, the high points of the 2 metal surfaces contact and wear away. This is boundary lubrication. Boundary lubrication is favored by heavy loads and low speeds.

In between these two regimes is the mixed lubrication.

The piston, ring and cylinder operate under conditions of hydrodynamic, mixed, and boundary lubrication depending on the load and operating speed.

A new engine has an imperfect fit of the ring to piston and cylinder wall. The ring has not yet achieved its best shape to act as a gas seal for hot combustion gases or to best transfer the heat from the piston to the cylinder wall.

The purpose of the break-in is to wear away the high spots on the ring and cylinder to give the best gas seal.

If the engine load is too high during this break-in, the ring is pushed toward boundary lubrication, which can tear away the metal by burnishing or galling. Once the metal is burnished, the gas sealing and heat transfer properties of the ring will never be optimum. The engine may run OK, but it won't run as well as it could have with a better break-in. Running an engine under high load during the break-in can also contribute to an increased potential for seizing. With a marginal gas seal, there is more blowby, more piston heat, and poorer heat transfer from the ring to the cylinder.

Light loads and a higher oil ratio during break-in push the rings and cylinder toward hydrodynamic and mixed lubrication. The wear on the rings occurs by micromachining away the high spots a little at a time. The excess oil also helps to flush away the wear particles. The light loads also minimize the temperature rise of the piston and minimize the amount of heat that has to be transfered from the ring to the cylinder. Once the ring, piston and cylinder are properly broken-in, the ring can make a good gas seal to minimize blowby, and can also transfer heat more effectively to the cylinder. Now the engine is ready to handle high loads, high power, and temperature.

If anyone has a problem with this information, please contact the Society of Automotive Engineers. I'm sure they would like to hear your theories.