*This applies to ALL engines, not just diesel.
The ultimate goal of a "break-in" period is to allow machined surfaces in the engine combustion chamber to conform to each other's shape. "Break-in" helps to creates a tight seal that is essential to preventing the escape of unburned fuel and pressurized gases from entering the crankcase. It also prevents engine lube oil from getting into the combustion chamber. It is perfectly reasonable and acceptable in new engines to see some small amount of engine oil consumed. While it is fair at first, it is essential that this loss of engine oil does not continue after the engine is broken-in.
For a break-in to occur, a moderate amount of heat, friction, and resulting wear will have to take place before the piston rings have “mated” with the cylinder walls. Running the generator engine under very light or no load at all will prevent the oil film on the cylinder wall from being scraped away by the piston rings. Eventually, this leaves a glaze (hard deposit) on the cylinder wall. Glazing prevents piston rings from creating a proper seal, and the result is a loss of power and engine lube oil. Contaminated oil will also result in increased engine wear.
Even with the most advanced engines with high precision parts, they still can't achieve a perfect fit without a proper break-in. The key to an adequate break-in is to apply a moderate load of about 75% of the rating of the engine so that there can be enough heat and pressure for the break-in to occur. The break-in can take from 100-200 hours.
When an engine is "breaking-in" there is a lot of heat from friction and wear that occurs. Be sure to do an oil and filter change sooner rather than later.
Also, try and avoid running the engine for long periods without sufficient loads as the decreased heat and pressure prevents the engine piston rings from wiping away oil and glazing may occur. Working under load increases the temperature and also causes the piston rings to expand to better fit and ware into the proper shape that will be needed when the engine is called upon to work hard in the future.