Our small town volunteer fire department is now split into two groups over a mechanical question. Myself and the one half want to put an air compressor system to keep the two air brake equipped trucks air systems charged. One is a 1991 American LaFrance with a 6V92 Detroit and a 1974 Chevy with a 427 gasoline engine. The other half believes these two trucks need the 3 to 4 minutes it takes to build up the air to warm up the engines. The Chevy, maybe, but I can't see the Detroit needing the warmup time (plus the 3-4 extra minutes headstart the fire gets on us due to this warmup time). Both engines are fairly low mileage and use Shell ROtell 15W-40 oil. Is there any validity to this warmup, or is the engine OK to go as soon as the oil pressure reaches normal?