2003 Ford F-150 Can idling ruin the engine?

Engine Performance problem
2003 Ford F150 Two Wheel Drive Automatic 35,000 miles

Our 2003 F150 HD is reported to need a new engine due to sludge. The truck is used by our secrurity staff that are in it 12 hours overnight and only put 25 miles driving, so it is constantly idling and sees no high speed road driving. It drove fine, with good acceleration up to 30,000 miles. After an idler arm repair the red engine light came on. We are told the idling, and too many (8-9K) miles between oil changes did it in. We have four or five security vehicles that idle overnight and want to avoid replacing engines. Can constant idling be so detrimental and what should we do to offset it? We did not see any warning on it in repair manuals. A lot of policecars idle for long periods.