I agree with your friend .. I feel that all engines regardless, of Diesel or not, need to be warm before they are placed under a heavy load ... the metal engine components need to be heated to work at the designed tolerances and the proper lubrication is occuring. I would say that if you were to start an engine then drive at a moderate speed without hard acceleration until it has that a chance to come up to operating temp. it would last a long time .. It's not only the engine lubrication that you should be concerned about but also the transmission, final drive etc. Long periods of idling also can be bad as not only are you wasting fuel, you are adding to carbon build up in the engine...there was a nice article on the Dodge / Cummins site about this ... Now if you live and work in Alaska (harsh cold climate) there is a need to idle more unless you have some way of heating the engine oil ... I guess my take on all of this is to be gentle with equipment untill it is warm and ready to go hard ... it's sorta like being an athlete, you just got to warm and stretch those muscles ...