One of my cousins owns a repair shop where he buys broken outboard motors and rebuilds them so that they work again. He has seen a lot of motors that are damaged because of regular wear and tear and other motors that have been damaged by owner negligence. Whenever he sells a motor, he gives the new owners very specific instructions about caring for their new motor.
He has a couple of new and used motors of his own for his different boats. Since these are all Yamaha outboard motors, he is careful to always use Yamalube oil and make sure there is always enough oil to keep the motor running smoothly. He is also very careful to ensure that he is using the right type of oil for each of his motors so that there are no problems.