We deal with Diesels every day, there are two schools of thought here, run them slower, low speed high torque, you don't have the same amount of oil pressure keeping your main and conrod bearings lubricated, we some time see crank damage due to people babying their machines, i don't know if that is them running is so slow that it constantly bogs down. The advantage is less fuel usage and in theory, longer life due to less sycles of the pistons in the bores.
The other side of it is, higher RPM you get better oil pressure, but you also burn more fuel and more piston cycles that will eventually wear the cylinders. In theory, less load on the bearings due to the higher speed of the pistons/con rods.
Ideally mid to high RPM is where you want it, but on my personal machine, i run it fairly low, but i don't work it really hard,