I'm with Clive on this one, old engines need gentle running in, but the new engines, are pretty much OK to GO from the start, my advice take it easy for 500 miles, change oil (gets rid of any small particles of metal) then go for it for 500 miles, oil change again, then give it hell. My view is that any small deposits from the engine can cause more wear than running the engine so keeping the oil spotless is my priority.
By the way I could pick up a brand new Tractor unit in the morning, straight under a loaded trailer (38 tonnes) and be in Scotland that evening, running in, who says.