5500 marquis diesel generator not started for years

I know that Cummings/Onan tells you to run every month. There are 2 reasons for this.

1. To keep the fuel fresh in the injection system
2. To keep moisture/condensation out of the generator.

Condensation can collect inside the generator and corrosion could be a problem. I would suggest that on an expensive piece of equipment like that to take it to a service center and give it a service before starting it. Tell them that it hasn't been started. It may prevent serious damage.