[Diesel_Dave]

I suspect the "maintenence" has to do with oil films too. Many of the internal parts (camshaft, crankshaft, etc.) are steel I assume. When an engine shuts down, a thin layer of oil is going to cover all the wear surfaces. As the engine sits, however, that oil will slowly drain down, eventually leaving spots where the metal surfaces will be unprotected, potentially opening the door for rust to occur. I suspect that's a major part of the "maintenence".

Machinery, in general, doesn't like to sit idle for a long period of time. You can take an old beater car and drive it to work day after day without problems, but park it for a couple months and then go try and start it and you'll be suprised the problems that often arise.